Document how to pass a 'decode' argument to get_payload when

is_multipart is False.

Doc/library/email.message.rst

@@ -112,6 +112,9 @@ Here are the methods of the :class:`Message` class:
       *decode* flag is ``True``, then ``None`` is returned.  The default for
       *decode* is ``False``.
 
+      To pass a value for the decode flag to a non-multipart message, specify
+      ``None`` as the value of *i*.
+
 
    .. method:: set_payload(payload[, charset])