Update doc for qos_policy_group NetApp extra spec

Bug 1288283 updated the function behind the qos_policy_group extra spec,
but the docs were never updated to reflect the change in behavior from
being a capability advertised to the scheduler to being a directive that
needed to be executed post provisioning.

Change-Id: Ia91a7cde1e0c9343de682d7b30a9898c6319ae8b
Merge "Update doc for qos_policy_group NetApp extra spec"