Document set_xauthrequest with pass_access_token (#829)

* Document set_xauthrequest with pass_access_token

Document feature implemented in https://github.com/oauth2-proxy/oauth2-proxy/pull/68

The feature is already decribed in in the nginx example but not clearly
on each respective parameters documentation.

* Update docs/configuration/configuration.md

Co-authored-by: Nick Meves <nick.meves@greenhouse.io>

Co-authored-by: Nick Meves <nick.meves@greenhouse.io>