api-ref: document microversions for the attachments API
api-ref: document microversions for the attachments API

The attachments API is completely new in the 3.27 API so
that should be pointed out in the API reference. The only
wrinkle is the 'complete attachment' API which is new in 3.44.

Change-Id: Ia22ddbcd86ecd12d74f365ec3c72d0635ff3566c
Partial-Bug: #1716705

openstack/cinder,phenoxim/cinder,eharney/cinder,mahak/cinder,openstack/cinder,Datera/cinder,Datera/cinder,j-griffith/cinder,phenoxim/cinder,mahak/cinder,eharney/cinder,j-griffith/cinder