[1.2.X] Fixed #13538 -- Clarified query examples with more explicit import...

[1.2.X] Fixed #13538 -- Clarified query examples with more explicit import statements and model vs. instance differentiation. Thanks to yipengh87 and kmtracey for the report, and timo for the patch.

Backport of [14070] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14073 bcc190cf-cafb-0310-a4f2-bffc1f526a37