Remove driver-specific notes from extend api-ref
Remove driver-specific notes from extend api-ref

This is not part of the API contract.  It should
be documented elsewhere as a limitation of the
volume driver.

Change-Id: I3586853ce7e11150d54cc552863bd3d4f6db197b

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