Added RemoteUserMiddleware to middleware reference page.
Added a note regarding django.contrib.auth.authenticate.

Thanks Josh Brown for the suggestion.
Fixed #22453 -- Corrected DoesNotExist message in docs.

Thanks Eugenio Cano-Manuel.
Fixed #22457 -- Fixed contributing guide w/o github set-up

The published commnand was not working when the user did not have
the git client set-up with the public-key. Changed the contributing
guide to clone it from https instead.
Fixed spelling errors in documentation.