Fixed #4991 -- Emphasized XSS ramifications of help_text not being escaped.

docs/ref/models/fields.txt

@@ -260,7 +260,9 @@ desire. For example::
     help_text="Please use the following format: <em>YYYY-MM-DD</em>."
 
 Alternatively you can use plain text and
-``django.utils.html.escape()`` to escape any HTML special characters.
+``django.utils.html.escape()`` to escape any HTML special characters. Ensure
+that you escape any help text that may come from untrusted users to avoid a
+cross-site scripting attack.
 
 ``primary_key``
 ---------------