Add REMOTE_USER mapping info in federation docs.

Reflect changes proposed in https://review.openstack.org/#/c/133037/ .
With the patch it is no longer compulsory to specify the rule for
``user_name`` mapping. If such rule is missing, a ``REMOTE_USER`` is
attempted.

Change-Id: I9d15720a533d68fe298e538ccaa0003464614347
Closes-Bug: #1390085
Merge "Add REMOTE_USER mapping info in federation docs."