[1.1.X] Fixed #13282 -- Clarified documentation around week_day filtering in...

[1.1.X] Fixed #13282 -- Clarified documentation around week_day filtering in querysets. Thanks to wangchun, Ramiro Morales and timo.

Backport of r13155 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13159 bcc190cf-cafb-0310-a4f2-bffc1f526a37