Fixed #16207 -- Enhanced documentation about user profile model instance...

Fixed #16207 -- Enhanced documentation about user profile model instance creation. Thanks foxwhisper for the report, melinath for the patch and Julien for reviewing it.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16450 bcc190cf-cafb-0310-a4f2-bffc1f526a37