PYTHONFSENCODING is not available on Windows or Mac OS X
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 45c1040..28df51d 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -447,6 +447,10 @@
If this is set before running the interpreter, it overrides the encoding used
for the filesystem encoding (see :func:`sys.getfilesystemencoding`).
+ This variable is not available (ignored) on Windows and Mac OS X: the
+ filesystem encoding is pinned to ``'mbcs'`` on Windows and ``'utf-8'`` on
+ Mac OS X.
+
.. versionadded:: 3.2