Add TBD about the (un)desirability of optional APIs.

pep-3156.txt

@@ -319,6 +319,10 @@ supports it).  The socket argument has to be a non-blocking socket.
   too slow for high-volume servers.  So there's also
   ``start_serving()`` above.  Then do we still need this?)
 
+TBD: Optional methods are not so good.  Perhaps these should be
+required?  It may still depend on the platform which set is more
+efficient.
+
 Callback Sequencing
 -------------------