Fixed #12193 - Add details to the i18n documentation for translation of model...

Fixed #12193 - Add details to the i18n documentation for translation of model classes (relations and methods). Thanks Maxime Petazzoni and Ramiro for work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15102 bcc190cf-cafb-0310-a4f2-bffc1f526a37