[1.2.X] Fixed #12193 - Add details to the i18n documentation for translation...

[1.2.X] 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.

Backport of r15102 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15103 bcc190cf-cafb-0310-a4f2-bffc1f526a37