Add pause/resume_writing, discard_output.

pep-3156.txt

@@ -1019,6 +1019,16 @@ Bidirectional stream transports have the following public methods:
 - ``resume()``.  Restart delivery of data to the protocol via
   ``data_received()``.
 
+- ``pause_writing()``.  Suspend sending data to the network until a
+  subsequent ``resume_writing()`` call.  Between ``pause_writing()``
+  and ``resume_writing()`` the transport's ``write()`` method will
+  just be accumulating data in an internal buffer.
+
+- ``resume_writing()``.  Restart sending data to the network.
+
+- ``discard_output()``.  Discard all data buffered by ``write()`` but
+  not yet sent to the network.
+
 - ``close()``.  Sever the connection with the entity at the other end.
   Any data buffered by ``write()`` will (eventually) be transferred
   before the connection is actually closed.  The protocol's