Fixed #13307 -- Enhanced the generic inlines documentation by mentioning the...

Fixed #13307 -- Enhanced the generic inlines documentation by mentioning the concrete Generic{Tabular,Stacked}Inline classes that should be used. Thanks loki77 for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15124 bcc190cf-cafb-0310-a4f2-bffc1f526a37