[IMP] purchase: update the units of measure page for V14 (#820)

The units of measure page needed an update to: match v14 visually (v12
screenshots), update the flow (the ratio works differently now and is
critical to understand), remove unnecessary information (e.g. 'install the
sales and purchase apps'), clarify some parts and correct typos

Co-authored-by: Xavier (xpl) <“xpl@odoo.com”>

purchase/products/uom.rst

@@ -1,133 +1,110 @@
 =================================================
-Purchase in different unit of measures than sales
+Purchase in different units of measure than sales
 =================================================
 
-In day-to-day business, it may happen that your supplier uses a
-different unit of measure than you do in sales. This can cause confusion
-between sales and purchase representative and even make you lose a lot
-of time converting measures. Luckily in Odoo, you can handle different
-units of measures between sales and purchase very easily.
+When you purchase a product, it may happen that your vendor uses a different unit of measure than
+you do when you sell it. This can cause confusion between sales and purchase representatives. It is
+also time-consuming to convert measures manually every time. With Odoo, you can configure your
+product once and let Odoo handle the conversion.
 
-Let's take the following examples:
+Consider the following examples:
 
-1)  You buy water from a supplier. The supplier is american and sells his
-    water in **Gallons**. Your customers however are European. You would
-    thus like to see your purchases quantities expressed in **Gallons**
-    and the sold quantities in **Liters**.
+#. You purchase orange juice from an American vendor, and they use **gallons**. However, your
+   customers are European and use **liters**.
 
-2)  You buy curtains from a supplier. The supplier sells you the curtains
-    in the unit **roll** and you sell the curtains in **square meters**.
+#. You buy curtains from a vendor in the form of **rolls** and you sell pieces of the rolls to your
+   customers using **square meters**.
 
-Configuration
-=============
+Enable units of measure
+=======================
 
-Install purchase and sales modules
-----------------------------------
+Open your Sales app and go to :menuselection:`Configuration --> Settings`. Under Product Catalog,
+enable *Units of Measure*.
 
-The first step is to make sure that the apps **Sales** and **Purchase** are
-correctly installed.
+.. image:: media/uom-enableoption.png
+   :align: center
 
-+------------+------------+
-|  |uom01|   |  |uom02|   |
-+============+============+
-+------------+------------+
+Specify sales and purchase units of measure
+===========================================
 
-Enable the Unit of Measures option
-----------------------------------
+Standard units of measure
+-------------------------
 
-Enter the purchase module, select :menuselection:`Configuration --> Settings` 
-and tick the **Some products may be sold/purchased in different unit of measures
-(advanced)** box.
-
-.. image:: media/uom08.png
-    :align: center
-
-Specify sales and purchase unit of measures
--------------------------------------------
-
-Standard units of measures
-~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Let's take the classic units of measures existing in Odoo as first
-example. Please remember that differents units of measures between sales
-and purchase necessarily need to share the same category. Categories
-include: **Unit**, **weight**, **working time**, **volume**, etc.
+A variety of units of measure are available by default in your database. Each belongs to one of the
+five pre-configured units of measure categories: *Length / Distance*, *Unit*, *Volume*, *Weight* and
+*Working Time*.
 
 .. tip::
-    It is possible to create your own category and unit of measure if 
-    it is not standard in Odoo (see next chapter).
+   You can create your new units of measure and units of measure categories (see next section).
 
-Let's assume we buy water from our vendors in **Gallons** and sell to our
-customers in **Liters**.
+To specify different units of measures for sales and purchases, open the Purchase app and go to
+:menuselection:`Products --> Products`. Create a product or select an existing one. Under the
+product's *General Information* tab, first select the *Unit of Measure* to be used for sales (as
+well as for other apps such as inventory). Then, select the *Purchase Unit of Measure* to be used
+for purchases.
 
-We go into the purchase module select :menuselection:`Purchase --> Products`.
+Back to the first example, if you purchase orange juice from your vendor in **gallons** and sell it
+to your customers in **liters**, first select *L* (liters) as the *Unit of Measure*, and *gal (US)*
+(gallons) as the *Purchase Unit of Measure*, then click on *Save*.
 
-Create your own product or select an existing one. In the products
-general information you have the possibility to select the **Unit of
-measure** (will be used in sales, inventory,...) and the **Purchase Unit
-of Measure** (for purchase).
+.. image:: media/uom-productconfiguration.png
+   :align: center
 
-In this case select **Liters** for **Unit of Measure** and **Gallons** for
-**Purchase Unit of Measure**.
+Create ynew units of measure and units of measure categories
+------------------------------------------------------------
 
-.. image:: media/uom05.png
-    :align: center
+Sometimes you need to create your own units and categories, either because the measure is not
+pre-configured in Odoo or because the units do not relate with each other (e.g. kilos and
+centimeters).
 
-Create your own unit of measure and unit of measure category
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If you take the second example where you buy curtains from a vendor in the form of **rolls** and you
+sell pieces of the rolls using **square meters**, you need to create a new *Units of Measure
+Category* in order to relate both units of measure.
 
-Let's take now our second example (you buy curtains from a supplier, the
-supplier sells you the curtains in the unit **roll** and you sell the
-curtains in **square meters**).
+To do so, go to :menuselection:`Configuration --> Units of Measure Categories`. Click on *Create*
+and name the category.
 
-The two measures are part of two different categories. Remember, you
-cannot relate an existing measure from one category with an existing
-measure of another category. We thus first have to create a shared
-**Measure Category** where both units have a conversion relationship.
+.. image:: media/uom-newcategory.png
+   :align: center
 
-To do so, go into your sales module select 
-:menuselection:`Configuration --> Products --> Unit of Measure`. 
-Create a new unit of **Measure Category** by selecting
-the dropdown list and clicking on create and edit (see picture below).
+The next step is to create the two units of measures. To do so, go to :menuselection:`Configuration
+--> Units of Measure`.
 
-.. image:: media/uom03.png
-    :align: center
+First, create the unit of measure used as the reference point for converting to other units of
+measure inside the category by clicking on *Create*. Name the unit and select the units of measure
+category you just created. For the *Type*, select *Reference Unit of Measure for this category
+type*. Enter the *Rounding Precision* you would like to use. The quantity computed by Odoo is always
+a multiple of this value.
 
-Create a new unit of measure. In this case our category will be called
-**Inter-Category-Computation**.
+In the example, as you cannot purchase less than 1 roll and won't use fractions of a roll as a unit
+of measure, you can enter 1.
 
-.. image:: media/uom06.png
-    :align: center
+.. image:: media/uom-newreferenceunit.png
+   :align: center
 
-The next step is to create the **Rolls** and **Square meter** units of
-measure and to link them to the new category. To do so, go into your
-purchase module select :menuselection:`Configuration --> Products --> Units of Measure`.
+.. note:: If you use a *Rounding Precision* inferior to 0.01, a warning message might appear stating
+   that it is higher than the *Decimal Accuracy* and that it might cause inconsistencies. If you
+   wish to use a *Rounding Precision* lower than 0.01, first :doc:`activate the Developer Mode
+   <../../general/developer_mode/activate>`, then go to :menuselection:`Settings --> Technical -->
+   Database Structure --> Decimal Accuracy`, select *Product Unit of Measure* and edit *Digits*
+   accordingly. For example, if you want to use a rounding precision of 0.00001, set *Digits* to 5.
 
-Create two new units:
+Next, create a second unit of measure, name it, and select the same units of measure category as
+your reference unit. As *Type*, select *Smaller* or *Bigger than the reference Unit of Measure*,
+depending on your situation.
 
--   The **Roll** unit who is part of the Inter-Category-Computation
-    category and is the **Reference Unit type** (see picture below). The
-    Reference Unit type is the measure set as a reference within the
-    category. Meaning that other measures will be converted depending
-    on this measure (ex: 1 roll = 10 square meters, 2 rolls = 20
-    square meters, etc.).
+As the curtain roll equals to 100 square meters, you should select *Smaller*.
 
-.. image:: media/uom04.png
-    :align: center
+Next, you need to enter the *Ratio* between your reference unit and the second one. If the second
+unit is smaller, the *Ratio* should be greater than 1. If the second unit is larger, the ratio
+should be smaller than 1.
 
--   For the **Square Meter**, we will specify that ``1 Roll = 10 square meters``
-    of curtain. It will thus be necessary to specify that as type,
-    the square meter is bigger than the reference unit. The **Bigger
-    Ratio** is ``10`` as ``one Roll = 10 square meters``.
+For your curtain roll, the ratio should be set to 100.
 
-.. image:: media/uom09.png
-    :align: center
+.. image:: media/uom-secondunit.png
+   :align: center
 
-It is now possible to input **square meters** as Unit of measure and a
-**Roll** as Purchase Unit of Measure in the product form.
+You can now configure your product just as you would using Odoo's standard units of measure.
 
-.. image:: media/uom07.png
-    :align: center
-
-.. |uom01| image:: ./media/uom01.png
-.. |uom02| image:: ./media/uom02.png
+.. image:: media/uom-productconfigurationnewunit.png
+   :align: center