Fixes #13538 -- Clarified query examples with more explicit import statements and model vs. instance differentiation. Thanks to yipengh87@gmail.com and kmtracey for the report, and timo for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14070 bcc190cf-cafb-0310-a4f2-bffc1f526a37