Fixed #15243 -- More clearly document that the transaction functions needs to...

Fixed #15243 -- More clearly document that the transaction functions needs to be called with a using parameter to work with a non-default database.  Thanks to Jason Kotenko for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15489 bcc190cf-cafb-0310-a4f2-bffc1f526a37