bpo-39759: Fixed os.getenv documentation

Doc/library/os.rst

@@ -217,7 +217,8 @@ 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. Only *key* needs to be a string as it is used to
+   lookup the value in os.environ.
 
    On Unix, keys and values are decoded with :func:`sys.getfilesystemencoding`
    and ``'surrogateescape'`` error handler. Use :func:`os.getenvb` if you

Misc/NEWS.d/next/Documentation/2020-02-26-15-21-17.bpo-39759.mMDBAl.rst

@@ -0,0 +1 @@
+Fixed os.getenv documentation.