Fixed #7284 -- Provided an example for the use of the search_fields clause on...

Fixed #7284 -- Provided an example for the use of the search_fields clause on ModelAdmin. Thanks to rbell01824@earthlink.net for the suggestion, and Simon Meers for the initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13542 bcc190cf-cafb-0310-a4f2-bffc1f526a37