[ADD] purchase: Lock a purchase order

https://docs.google.com/document/d/18ure1GXwOfgohvbC0ybkMyYwgLOJ2fjsUAu08Fk26y8/edit

purchase/purchases/rfq.rst

@@ -1,6 +1,6 @@
-======================
-Request for Quotations
-======================
+==========================================
+Request for Quotations and Purchase Orders
+==========================================
 
 .. toctree::
    :titlesonly:
@@ -8,6 +8,7 @@ Request for Quotations
    rfq/create
    rfq/bills
    rfq/reception
+   rfq/lock_orders
    rfq/cancel
    rfq/approvals
    rfq/analyze

purchase/purchases/rfq/lock_orders.rst

@@ -0,0 +1,28 @@
+=====================
+Lock a purchase order
+=====================
+
+Once an order is confirmed, you can lock it preventing any further
+editing.
+
+Configuration
+=============
+
+For this feature to work, go to :menuselection:`Purchases -->
+Configuration --> Settings` and activate the *Lock Confirmed Orders*
+feature
+
+.. image:: media/lock_orders01.png
+    :align: center
+
+Lock an order
+=============
+
+While working on a purchase order, when you confirm it, you can then
+lock the order preventing any further modification on the document.
+
+.. image:: media/lock_orders02.png
+    :align: center
+
+.. note::
+    You can unlock the document if you need to make additional changes