Improve messages api-ref description
Improve messages api-ref description

The description for the messages API was very terse. This adds some more
details to provide some description of what the API is used for.

Change-Id: If15400e9a538bf533e93f9693b6eddeb114fa7e5
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

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