documentation does not explain how http/s rule URLs are built

[Doug Hellmann]

It turns out that values from the target of a policy check can be interpolated into the URL value to build a custom URL. For example, if target includes "resource_type" with a value of "my_resource" and the rule is https://examples.com/%(resource_type)s then the URL for the check is https://exmples.com/my_resource.

We need to document that.

It would be nice if we could document some common values from the target dicts as well.

[Doug Hellmann (doug-hellmann)]

See https://opendev.org/openstack/oslo.policy/src/branch/master/oslo_policy/_external.py#L36 and https://opendev.org/openstack/oslo.policy/src/branch/master/oslo_policy/_external.py#L75 for the relevant implementations.