Fix docstrings in federation related modules

Updated docstrings in keystone/auth/plugins/saml2.py and
keystone/contrib/federation/routers.py, included additional
parameters there was previously missing.

Change-Id: Id8355cdc1ab80b57ed18fcb3b16fd92cb7a73ecf
Closes-Bug: #1288124
Merge "Fix docstrings in federation related modules"