gh-85133: Add that getenv uses os.environ

Doc/library/os.rst

@@ -292,7 +292,10 @@ process and user.
 .. function:: getenv(key, default=None)
 
    Return the value of the environment variable *key* if it exists, or
-   *default* if it doesn't. *key*, *default* and the result are str.
+   *default* if it doesn't. *key*, *default* and the result are str. Note that
+   since :func:`getenv` uses :data:`os.environ`, the mapping of :func:`getenv` is
+   similarly also captured on import, and the function may not reflect
+   future environment changes.
 
    On Unix, keys and values are decoded with :func:`sys.getfilesystemencoding`
    and ``'surrogateescape'`` error handler. Use :func:`os.getenvb` if you
@@ -304,7 +307,11 @@ process and user.
 .. function:: getenvb(key, default=None)
 
    Return the value of the environment variable *key* if it exists, or
-   *default* if it doesn't. *key*, *default* and the result are bytes.
+   *default* if it doesn't. *key*, *default* and the result are bytes. Note that
+   since :func:`getenvb` uses :data:`os.environb`, the mapping of :func:`getenvb` is
+   similarly also captured on import, and the function may not reflect
+   future environment changes.
+
 
    :func:`getenvb` is only available if :data:`supports_bytes_environ`
    is ``True``.
@@ -514,7 +521,8 @@ process and user.
    Assignments to items in ``os.environ`` are automatically translated into
    corresponding calls to :func:`putenv`; however, calls to :func:`putenv`
    don't update ``os.environ``, so it is actually preferable to assign to items
-   of ``os.environ``.
+   of ``os.environ``. This also applies to :func:`getenv` and :func:`getenvb`, which
+   respectively use ``os.environ`` and ``os.environb`` in their implementations.
 
    .. note::