Fixed #10142 -- Added docs and an exploding error message to highlight an...

Fixed #10142 -- Added docs and an exploding error message to highlight an error present in the implementation of STDDEV_POP and VAR_POP in PostgreSQL 8.2-8.2.4 that will give incorrect answers (the database function mistakenly returns sample, rather than population deviation/variance). Thanks to Vinay Sajip <vinay_sajip@yahoo.co.uk> for the report, and Ian Kelly for pointing out the cause of the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9804 bcc190cf-cafb-0310-a4f2-bffc1f526a37