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.

Cherry-picked from master:
https://review.openstack.org/#/c/122302/

In the above commit (https://review.openstack.org/#/c/122302/), I
didn't follow the doc guidelines and used the term Cinder. This fixes
this snafoo.

Cherry-picked from master:
https://review.openstack.org/#/c/122487/

Change-Id: I226c58c112e8485951834978b4fb971edaad5d81
(cherry picked from commit f6357f2fca0a1bcb526af80a3e52e6adf336b1d7)