Fixed #16392 -- Clarified docs of the "c" date format with regard to naive...

Fixed #16392 -- Clarified docs of the "c" date format with regard to naive datetime objects. Thanks, ybon.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16507 bcc190cf-cafb-0310-a4f2-bffc1f526a37