Fixes wrong value description for volume-detach

The description of the <volume> argument is not proper, the <volume>
argument is indeed "ID of the volume". “Attachment ID" is the internal
term of nova/cinder. So change description 'Attachment ID of the
volume.' to 'ID of the volume to detach.'

Change-Id: I262d72f6f676e260a58d45b750dbd60fc7c3dcf5
Closes-bug: 1321073
Merge "Fixes wrong value description for volume-detach"