[1.2.X] Fixed #15365 -- Added a warning to the `contrib.markup` docs reminding...

[1.2.X] Fixed #15365 -- Added a warning to the `contrib.markup` docs reminding users that the marked up output will not be escaped.

Backport of [15673] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15674 bcc190cf-cafb-0310-a4f2-bffc1f526a37