Doc'd that assertXMLEqual()/assertXMLNotEqual() ignores XML declaration and comments.

docs/topics/testing/tools.txt

@@ -1645,6 +1645,9 @@ your test suite.
     syntax differences. When invalid XML is passed in any parameter, an
     ``AssertionError`` is always raised, even if both string are identical.
 
+    XML declaration and comments are ignored. Only the root element and its
+    children are compared.
+
     Output in case of error can be customized with the ``msg`` argument.
 
 .. method:: SimpleTestCase.assertXMLNotEqual(xml1, xml2, msg=None)