api-ref: volume summary is available from v3.12
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.

Change-Id: I7211ba4355fe9a10203e01436e571f41f8040adc

mahak/cinder,mahak/cinder,openstack/cinder,openstack/cinder