bpo-13601: always use line-buffering for sys.stderr (GH-17646)
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index a824fb9..0aae263 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1446,9 +1446,15 @@
for the Windows console, this only applies when
:envvar:`PYTHONLEGACYWINDOWSSTDIO` is also set.
- * When interactive, ``stdout`` and ``stderr`` streams are line-buffered.
- Otherwise, they are block-buffered like regular text files. You can
- override this value with the :option:`-u` command-line option.
+ * When interactive, the ``stdout`` stream is line-buffered. Otherwise,
+ it is block-buffered like regular text files. The ``stderr`` stream
+ is line-buffered in both cases. You can make both streams unbuffered
+ by passing the :option:`-u` command-line option or setting the
+ :envvar:`PYTHONUNBUFFERED` environment variable.
+
+ .. versionchanged:: 3.9
+ Non-interactive ``stderr`` is now line-buffered instead of fully
+ buffered.
.. note::