Fixed #20197 -- Made XML serializer fail loudly when outputting unserializable chars

Thanks Tim Graham for the review.

docs/topics/serialization.txt

@@ -213,6 +213,16 @@ the auth.User model has such a relation to the auth.Permission model::
 
 This example links the given user with the permission models with PKs 46 and 47.
 
+.. admonition:: Control characters
+
+    .. versionchanged:: 1.9
+
+    If the content to be serialized contains control characters that are not
+    accepted in the XML 1.0 standard, the serialization will fail with a
+    :exc:`ValueError` exception. Read also the W3C's explanation of `HTML,
+    XHTML, XML and Control Codes
+    <http://www.w3.org/International/questions/qa-controls>`_.
+
 .. _serialization-formats-json:
 
 JSON