Fixed #11931: Removed mention of nonexistent get_sql() method for arguments to...

Fixed #11931: Removed mention of nonexistent get_sql() method for arguments to limit_choices_to. Since the correct reference involves undocumented ORM internals, this simply removes the reference entirely in favor of publicly-documented use of Q objects.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11591 bcc190cf-cafb-0310-a4f2-bffc1f526a37