Add policy in code documentation for os-set_bootable API
Add policy in code documentation for os-set_bootable API

This patchset adds policy in code documentation for the
os-set_bootable API action, which does not directly [0]
enforce any policy. However, it does internally call volume
update API via self.volumes_api.update() [1] which in turn
enforces the update policy [2]. Thus os-set_bootable indirectly
enforces this policy.

This is a unique case where an API doesn't enforce something
directly, yet where policy still matters for the API to pass
successfully.

[0] https://github.com/openstack/cinder/blob/52deeff8d2519c1700aa880d3fae0bb0e83ea86c/cinder/api/contrib/volume_actions.py#L310
[1] https://github.com/openstack/cinder/blob/52deeff8d2519c1700aa880d3fae0bb0e83ea86c/cinder/api/contrib/volume_actions.py#L321
[2] https://github.com/openstack/cinder/blob/52deeff8d2519c1700aa880d3fae0bb0e83ea86c/cinder/volume/api.py#L535

Change-Id: I29d54fb391f4e9508d86fc93e84b4b02c1333da0

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