Update usage documentation
Update usage documentation

We recently merged a patch that enhanced the RuleDefault object by
allowing it to have more attributes:

  Ie9b335420394166bb39c43e3d26fcc9237ffd1a0

This commit describes its usage in the usage documentation.

Change-Id: I8f6e6bc289a046b09529f707874314c69757ee11

openstack/oslo.policy