[ADD] purchase: Add blanket order documentation

* Add chapter documenting the use of Blanket Order type Purchase Agreements
* Clarify the proper usage and behavior of dates for Purchase Tender type Purchase Agreements

purchase/purchases/tender.rst

@@ -6,4 +6,5 @@ Purchase Tenders
    :titlesonly:
 
    tender/manage_multiple_offers
-   tender/partial_purchase
+   tender/partial_purchase
+   tender/manage_blanket_orders

purchase/purchases/tender/manage_blanket_orders.rst

@@ -0,0 +1,88 @@
+============================
+How to manage Blanket Orders
+============================
+
+A **Blanket Order** is a contract between you (the customer) and your supplier. 
+It is used to negotiate a discounted price. The supplier is benefited by the
+economies of scale inherent in a large order. You are benefited by being allowed
+to take multiple smaller deliveries over a period of time, at a lower price,
+without paying for the large order immediately. Each small periodic delivery
+is called a release or call-off.
+
+As the blanket order is a contract, it will have some prearranged conditions.
+These usually include:
+
+-   Total quantity of each product to be delivered
+
+-   Completion deadline, by which you must take delivery of the total quantity
+
+-   Unit price for each product
+
+-   Delivery lead time in days for each release
+
+Activate the Purchase Agreements
+--------------------------------
+
+The Blanket Order function is provided by the Purchase Agreements feature. 
+By default, the Purchase Agreements is not activated. To be able to use blanket orders,
+you must first activate the option.
+
+In the Purchases module, open the Configuration menu and click on
+Settings. In the **Orders** section, locate the **Purchase Agreements**
+and tick the box, then click on **Save**.
+
+.. image:: media/manage_multiple_offers00.png
+	:align: center
+
+Create a Blanket Order
+----------------------
+
+To create a new blanket order, open :menuselection:`Purchase --> Purchase Agreements`.
+
+.. image:: media/manage_multiple_offers01.png
+	:align: center
+
+In the Purchase Agreements window, click on **Create**. A new Purchase
+Agreement window opens.
+
+In the **Agreement Type** field, choose Blanket Order.
+
+Choose the **Vendor** with whom you will make the agreement.
+
+Set the **Agreement Deadline** as per the conditions of the agreement.
+
+Set the **Ordering Date** to the starting date of the contract.
+
+Leave the **Delivery Date** empty because we will have different delivery dates
+with each release.
+
+In the **Products** section, click on **Add an item**. Select products
+in the Product list, then insert **Quantity**. You can add as many
+products as you wish.
+
+.. image:: media/manage_multiple_offers02.png
+	:align: center
+
+Click on **Confirm**.
+
+Now click on the button **New Quotation**. A RfQ is created for this vendor, with the
+products chosen on the PT. Repeat this operation for each release.
+
+Be careful to change the **Quantity** field on the RFQ.  By default, the RFQ quantity
+will be for the entire remaining quantity. Your individual releases should be for some smaller
+quantity.
+
+.. image:: media/manage_multiple_offers03.png
+	:align: center
+
+When all of the releases (purchase orders) have been delivered and paid, you can click on
+**Validate** and **Done**.
+
+View `Purchase Agreements <https://demo.odoo.com/?module=purchase_requisition.action_purchase_requisition>`__
+in our Online Demonstration.
+
+.. seealso:: 
+
+	:doc:`../../overview/process/from_po_to_invoice`
+
+	:doc:`../../overview/process/difference`

purchase/purchases/tender/manage_multiple_offers.rst

@@ -41,6 +41,12 @@ Agreement window opens.
 
 In the **Agreement Type** field, choose Purchase Tender.
 
+The **Agreement Deadline** field tells the vendors when to have their offers submitted.
+
+The **Ordering Date** field tells the vendors when we will submit a purchase order to the chosen vendor.
+
+The **Delivery Date** field tells the vendors when the product will have to be delivered.
+
 You do not have to define a **Vendor**.
 
 In the **Products** section, click on **Add an item**. Select products