[1.1.X] Fixed #11889: Documented the fact that context processors will...

[1.1.X] Fixed #11889: Documented the fact that context processors will overwrite pre-existing context variables of the same names. Backport of [12669] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12670 bcc190cf-cafb-0310-a4f2-bffc1f526a37