asyncio: update the doc

* dev: mention that the logging must be configured at DEBUG level
* streams: drain() has no more a strange return value, it's just
  a standard coroutine

--HG--
branch : 3.4
asyncio: sync with Tulip

* PipeServer.close() now cancels the "accept pipe" future which cancels the
  overlapped operation.
* Fix _SelectorTransport.__repr__() if the transport was closed
* Fix debug log in BaseEventLoop.create_connection(): get the socket object
  from the transport because SSL transport closes the old socket and creates a
  new SSL socket object. Remove also the _SelectorSslTransport._rawsock
  attribute: it contained the closed socket (not very useful) and it was not
  used.
* Issue #22063: socket operations (sock_recv, sock_sendall, sock_connect,
  sock_accept) of the proactor event loop don't raise an exception in debug
  mode if the socket are in blocking mode. Overlapped operations also work on
  blocking sockets.
* Fix unit tests in debug mode: mock a non-blocking socket for socket
  operations which now raise an exception if the socket is blocking.
* _fatal_error() method of _UnixReadPipeTransport and _UnixWritePipeTransport
  now log all exceptions in debug mode
* Don't log expected errors in unit tests
* Tulip issue 200: _WaitHandleFuture._unregister_wait() now catchs and logs
  exceptions.
* Tulip issue 200: Log errors in debug mode instead of simply ignoring them.

--HG--
branch : 3.4
asyncio, tulip issue 203: Add _FlowControlMixin.get_write_buffer_limits() method

--HG--
branch : 3.4
extra : amend_source : 5f391478793b30ad02480a1e642330e91003e39b
Issue #22063: Fix asyncio documentation of socket and pipe operations regarding
to non-blocking mode. The non-blocking mode does not matter when using a
ProactorEventLoop, only for SelectorEventLoop.

--HG--
branch : 3.4