document how to deal with binary standard streams
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index e4dbf17..e87b7e1 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -709,6 +709,12 @@
: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__