[ADD] purchase: How to cancel a Purchase Order?

https://docs.google.com/document/d/13lt1LTsXIUz6bn3gyFC-k3Th9xsZjp0scchozukf5K8/edit#heading=h.teahchhagxca

purchase/purchases/rfq/cancel.rst

@@ -1,3 +1,105 @@
 ================================
 How to cancel a purchase order?
-================================
+================================
+
+Due to misunderstandings, human errors or change of plans, it is
+sometimes necessary to cancel purchase orders sent to suppliers. Odoo
+allows you to do it, even if some or even all of the ordered goods
+already arrived in your warehouse.
+
+We will first take as example the case where you order **3 iPad mini** that
+haven't arrived in your transfers yet. As the installation of the
+inventory application is required when using the **Purchase** module, it is
+also interesting to see the case of partially delivered goods that you
+want to cancel.
+
+Create a Purchase Order
+=======================
+
+The first step to create a **Purchase Order** is to create a **Request for
+Quotation (RFQ)** from the menu 
+:menuselection:`Purchases --> Purchase --> Requests for quotation`. 
+Confirm your RFQ to have a confirmed purchase order
+
+.. note:: 
+    To learn more about the purchase order process, 
+    read the documentation page 
+    :doc:`../../overview/process/from_po_to_invoice`
+
+Cancel your Purchase Order
+==========================
+
+Use case 1 : you didn’t receive your goods yet
+----------------------------------------------
+
+If you confirmed your purchase order and did not received your goods
+yet, you can simply cancel the PO it by clicking the cancel button.
+
+.. image:: media/cancel05.png
+    :align: center
+
+Odoo will automatically cancel the outstanding shipments related to this
+PO and the status bar will switch from **Purchase order** to **Cancelled**.
+
+.. image:: media/cancel03.png
+    :align: center
+
+Use case 2 : partially delivered goods
+--------------------------------------
+
+In this case, **2** of the **3 iPad Mini** arrived before you needed to cancel
+the PO.
+
+Register good received and cancel backorder
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The first thing to do will be to register the goods received and to
+cancel the arrival of the **third iPad Mini** that is still supposed to be
+shipped. From the PO, click on **Receive products** and, on the **iPad Mini
+order line**, manually change the received quantities under the Column
+**Done**.
+
+.. image:: media/cancel02.png
+    :align: center
+
+.. note::
+    To learn more, see :doc:`reception`
+
+When clicking on **Validate**, Odoo will warn you that you have processed
+less products than the initial demand (2 instead of 3 in our case) and
+will ask you the permission to create a backorder.
+
+.. image:: media/cancel07.png
+    :align: center
+
+Click on **No backorder** to cancel the supply of the remaining product.
+You will notice than the quantity to receive has been changed
+accordingly and, therefore, the delivery status has switched to **Done**.
+
+.. image:: media/cancel04.png
+    :align: center
+
+Create reverse transfer
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Now, you need to return the iPad Minis that you have received to your
+vendor location. To do so, click on the **Reverse** button from the same
+document. A reverse transfer window will pop up. Enter the quantity to
+return and the corresponding location and click on **Return**.
+
+.. image:: media/cancel06.png
+    :align: center
+
+Process the return shipment and control that the stock move is from your
+stock to your vendor location.
+
+.. image:: media/cancel01.png
+    :align: center
+
+When the reverse transfer is done, the status of your purchase order
+will be automatically set to done, meaning that your PO has been
+completely cancelled.
+
+.. seealso::
+    * :doc:`bills`
+    * :doc:`reception`