gh-101754: Document that Windows converts keys in `os.environ` to uppercase (GH-101840)

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 85924d0e48366bed96fcd2bc722f0630eab1eddb..23ce98785bedfc6d137f606459eb38002c704250 100644 (file)
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -201,6 +201,11 @@ process and user.
    ``'surrogateescape'`` error handler. Use :data:`environb` if you would like
    to use a different encoding.
 
+   On Windows, the keys are converted to uppercase. This also applies when
+   getting, setting, or deleting an item. For example,
+   ``environ['monty'] = 'python'`` maps the key ``'MONTY'`` to the value
+   ``'python'``.
+
    .. note::
 
       Calling :func:`putenv` directly does not change :data:`os.environ`, so it's better