Fixed #12642 - Add docs for has_[add|change|delete]_permission ModelAdmin...

Fixed #12642 - Add docs for has_[add|change|delete]_permission ModelAdmin methods. Thanks to MadeR for the report and for the wiki contributors from which I took language for this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15066 bcc190cf-cafb-0310-a4f2-bffc1f526a37