[1.3.X] Refs #15855 -- Recommended the csrf_protect decorator rather than...

[1.3.X] Refs #15855 -- Recommended the csrf_protect decorator rather than vary_on_cookie as workaround for cache_page caching the response before it gets to middleware.

Backport of r16361 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16362 bcc190cf-cafb-0310-a4f2-bffc1f526a37