Fixed #23602 -- Add comment on get_absolute_url regarding user input

https://code.djangoproject.com/ticket/23602
This is looking good to me! One remaining comment: in many cases valid URLs will be built from user input (e.g. article slugs or something), and that's fine as long as that user input is properly validated. I don't know that we need to go into detail about that, but I would just say "You should avoid building the URL from un-validated user input" instead of just "from user input".