Fixed #16079: Clarified (for real this time) how handler404 and handler500...

Fixed #16079: Clarified (for real this time) how handler404 and handler500 work, and that they only work in a root URLconf.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16804 bcc190cf-cafb-0310-a4f2-bffc1f526a37