Added comment to UserAdmin.add_view() explaining why we disallow users without...

Added comment to UserAdmin.add_view() explaining why we disallow users without change permissions from adding other users. Refs #9866

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9682 bcc190cf-cafb-0310-a4f2-bffc1f526a37