Add YAML format into docs for policy file

kolla-ansible support yaml format for policy file[1] too, but the
docs missing it, this ps to add it.

[1]: https://github.com/openstack/kolla-ansible/blob/master/ansible/group_vars/all.yml#L393
Closes-Bug: #1804455

Change-Id: I44eb1d64f9299ccaf99972c8b5354683a3501f6b