Update Block Storage API v2 api-ref

Add a note that the v2 API was removed in Xena.

Change-Id: I3dd81d81995913e30addf5bf4ee8a4406c36632f

api-ref/source/v2/index.rst

@@ -1,8 +1,23 @@
 :tocdepth: 2
 
-=================================
-Block Storage API V2 (DEPRECATED)
-=================================
+==============================
+Block Storage API V2 (REMOVED)
+==============================
+
+.. note::
+    Version 2 of the Block Storage API was `deprecated in the Pike release
+    <https://docs.openstack.org/releasenotes/cinder/pike.html#deprecation-notes>`_
+    and was removed during the Xena development cycle.  `This document is
+    maintained for historical purposes only.`
+
+    `Version 3
+    <https://docs.openstack.org/api-ref/block-storage/v3/>`_
+    of the Block Storage API was `introduced in the Mitaka release
+    <https://review.opendev.org/c/openstack/cinder/+/224910>`_.  Version
+    3.0, which is the default microversion at the ``/v3`` endpoint, was
+    designed to be identical to version 2.  Thus, scripts using the Block
+    Storage API v2 should be adaptable to version 3 with minimal changes.
+
 
 .. rest_expand_all::