[ADD] purchase: Lock a purchase order

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

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