pythongh-101754: Document that Windows converts keys in os.environ …

…to uppercase (pythonGH-101840)

(cherry picked from commit 4e7c0cb)

Co-authored-by: Owain Davies <116417456+OTheDev@users.noreply.github.com>

Doc/library/os.rst

@@ -188,6 +188,11 @@ 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