Corrected explanation of values() and extra() interaction in querysets.

Fixes #15546, refs #13455. The original documentation patch
inadvertently muddied the waters in one aspect and this commit tidies
that up.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16697 bcc190cf-cafb-0310-a4f2-bffc1f526a37