Reference docstring for auth_token fields

The documented variables that the auth_token middleware sets for
the request was out of date with the code. Rather than maintain
the documentation for the fields in 2 places we'll now link to the
auth_token module docs.

Change-Id: Id4ca1843f749c1ed0b515c8860764add3ec4a3da
Merge "Reference docstring for auth_token fields"