document how to deal with binary standard streams

Doc/library/sys.rst

@@ -709,6 +709,12 @@ always available.
    :func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
    the :mod:`os` module.)
 
+   .. note::
+
+      The standard streams are text in text mode by default.  To write or read
+      binary data to these, use the underlying binary buffer.  For example, to
+      write bytes to :data:`stdout`, use ``sys.stdout.buffer.write(b'abc')``.
+
 
 .. data:: __stdin__
           __stdout__