docs need more detail about HTTP check

[Doug Hellmann]

The discussion of HTTP Check at https://docs.openstack.org/oslo.policy/latest/reference/api/oslo_policy.policy.html#http-check does not explain how the values are passed to the remote server. Are they form encoded? Is the body of the HTTP request a JSON document? Is the URL accessed via GET or POST? Exactly how is the response evaluated to determine if the check passes or not?