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

https://code.djangoproject.com/ticket/23602
I don't think the contenttypes framework or its shortcut view are deserving of specific mention here. Link poisoning and redirect poisoning are both bad; the warning here would still stand even if the contenttypes redirect view did not exist. Also, I think this warning is just too long. So I would cut this entire paragraph and replace it with simply the example of what you should not do, and what type of malicious link it can result in.