Fixed #13418 -- Added notes on uniqueness requirements for natural keys....

Fixed #13418 -- Added notes on uniqueness requirements for natural keys. Thanks to hunajakippo for the suggestion, and Ramiro Morales for the draft text.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13146 bcc190cf-cafb-0310-a4f2-bffc1f526a37