[1.1.X] Fixed #13253 -- Updated the help message for dumpdata to indicate that...

[1.1.X] Fixed #13253 -- Updated the help message for dumpdata to indicate that individual models can be serialized. Thanks to arowla for the report.

Backport of r12893 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12894 bcc190cf-cafb-0310-a4f2-bffc1f526a37