[1.2.X] Fixed #11594 - Inaccurate docstring for WhereNode.add()

Thanks to garrison for report, dwillis for patch

Backport of [13843] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13844 bcc190cf-cafb-0310-a4f2-bffc1f526a37