Scope federated tokens with ``token`` auth method.

For a greater unification, process of scoping federated tokens should
use ``token`` auth method instead of ``saml2``.
Required code changes are in https://review.openstack.org/#/c/130593
This patch reflects the changes in the documentation.

Change-Id: Ieafec9c9c8d277061b181df15926b5330b87c578
Merge "Scope federated tokens with ``token`` auth method."