Added `using` to the list of documented arguments for save() on a model;...

Added `using` to the list of documented arguments for save() on a model; updated the docs to suggest using ``*args, **kwargs`` when implementing model save methods. Thanks to Jeff Croft for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12118 bcc190cf-cafb-0310-a4f2-bffc1f526a37