[1.3.X] Fixed #16904 -- Additional clarification regarding contrib.messages...

[1.3.X] Fixed #16904 -- Additional clarification regarding contrib.messages iteration. Thanks murphyke for the report and patch.

Backport of [16866] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16867 bcc190cf-cafb-0310-a4f2-bffc1f526a37