update documentation for X.509 tokenless auth
update documentation for X.509 tokenless auth

Explain what this feature is intended for and how to properly
use it.

Change-Id: I5ef67d9beaa0fc9505270408db4dec5dd9d97ebf
Closes-Bug: 1813057

openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone,openstack/keystone,mahak/keystone