Make fd methods optional.

pep-3156.txt

@@ -175,6 +175,11 @@ A conforming event loop object has the following methods:
   later time in a threadsafe manner, you can use
   ``ev.call_soon_threadsafe(ev.call_later, when, callback, *args)``.)
 
+The following methods for registering callbacks for file descriptors
+are optional.  The default implementation provides them but the user
+normally doesn't use these directly -- they are used by the transport
+implementations exclusively:
+
 - ``add_reader(fd, callback, *args)``.  Arrange for
   ``callback(*args)`` to be called whenever file descriptor ``fd`` is
   ready for reading.  Returns a ``DelayedCall`` object which can be