Merge #11780: document that email.encoders throw TypeError on multipart messages.

Doc/library/email.encoders.rst

@@ -18,6 +18,10 @@ exactly one argument, the message object to encode.  They usually extract the
 payload, encode it, and reset the payload to this newly encoded value.  They
 should also set the :mailheader:`Content-Transfer-Encoding` header as appropriate.
 
+Note that these functions are not meaningful for a multipart message.  They
+must be applied to individual subparts instead, and will throw a
+:exc:`TypeError` if passed a message whose type is multipart.
+
 Here are the encoding functions provided: