Bunch of improvements on authentication documentation

[mkniewallner]

AuthenticationBackend.authenticate signature was updated here to handle both HTTP and websocket protocols, but the documentation still uses request.

Also removing UnauthenticatedUser import in AuthenticationBackend example, as it is not used, and adding some missing imports in a few examples.

Side-note: The documention mixes single and double quotes usage for strings. Should a consistent style be used? Maybe https://github.com/asottile/blacken-docs could be helpful?

[Kludex]

Side-note: The documention mixes single and double quotes usage for strings. Should a consistent style be used? Maybe https://github.com/asottile/blacken-docs could be helpful?

I recommend a GitHub discussion or introducing the idea on our Gitter server.

Thanks for the PR! 🎉