Adds the v2 and v3 API reference documentation for the
admin-only (by default) os-migrate_volume volume action
API. The only major difference with the v3 API is the
cluster parameter introduced in the 3.16 microversion.
Change-Id: I70f6b2cc4d25ea155ce09ddeec26b995890a2db3
Partial-Bug: #1607539