[1.3.X] Fixed #17837. Improved markdown safety.

Markdown enable_attributes is now False when safe_mode is enabled. Documented
the markdown "safe" argument. Added warnings when the safe argument is
passed to versions of markdown which cannot be made safe.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17734 bcc190cf-cafb-0310-a4f2-bffc1f526a37