Fixed #10574 -- Documented interaction between annotations and order_by.

In the future, I'd like to fix this properly, but the current behavior
has the advantage of being consistent across the board (and changing it
everywhere is backwards-incompatible with documented functionality).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10172 bcc190cf-cafb-0310-a4f2-bffc1f526a37