api-ref: volume summary is available from v3.12
GET volumes/summary is introduced to Cinder from V3.12, so this
to add the comment for it.
@@ -759,7 +759,8 @@ Get volumes summary
.. rest_method:: GET /v3/{project_id}/volumes/summary
-Display volumes summary with total number of volumes and total size in GB
+Display volumes summary with total number of volumes and total size in GB.
+Available since API microversion 3.12.
Response codes
