[1.6.x] Added RemoteUserMiddleware to middleware reference page.

Backport of 465980d070a33555cdabb4787db50a4e23228de2 from master.
[1.6.x] Added a note regarding django.contrib.auth.authenticate.

Thanks Josh Brown for the suggestion.

Backport of ae5b662796 from master
[1.6.x] Fixed #22453 -- Corrected DoesNotExist message in docs.

Thanks Eugenio Cano-Manuel.

Backport of eefad142dee466c1ead6038fbeec75479b1afead from master
[1.6.x] 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.

Backport of 28102991172ab931d030412275f926c1b05c688d from master.
[1.6.x] Fixed #22220 -- Added more examples to reverse() documention.

Thanks EvilDMP for the suggestions.

Backport of 030dd4f72c from master