[1.6.x] Added a warning regarding risks in serving user uploaded media.

Thanks Preston Holmes for the draft text.

Backport of df6760f12c from master

docs/ref/settings.txt

@@ -1440,6 +1440,12 @@ to a non-empty value.
 
 Example: ``"http://media.example.com/"``
 
+.. warning::
+
+    There are security risks if you are accepting uploaded content from
+    untrusted users! See the security guide's topic on
+    :ref:`user-uploaded-content-security` for mitigation details.
+
 .. warning::
 
     :setting:`MEDIA_URL` and :setting:`STATIC_URL` must have different