modifying the request object with middleware

[mpede]

There is a cool feature where if you add a urlconf property to the request object passed to your process_request function of your middleware object it will look to that property to determine what gets passed to the URL resolver.

This is really nice to take advantage of, but perhaps it deserves to be a documented feature of something custom Middleware can do. Documenting this and perhaps other things like it I haven't found would really be useful for some people.
I'd also like to know that this feature isn't going to go away. Documenting it would definitely make this concern go away.

Thanks a lot for your work on Django! I'd like to find a way for our workplace to contribute back to this project.

[Malcolm Tredinnick]

Dupe of #6382.