Clarify volume migration CLI reference
Clarify volume migration CLI reference

Wording for the volume migration was not very clear. This reworks
that text to be a little more concise. It also removes an OSC arg
that does not actually do anything.

Change-Id: Id75bd6b4445fcf8042d87578f5f692958a859c0b

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