gh-83940: os docs: Improve wording for getenv/getenvb (GH-98113)

(cherry picked from commit 187e853)

Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>

Author: miss-islington

Doc/library/os.rst

@@ -291,8 +291,8 @@ 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. Note that
+   Return the value of the environment variable *key* as a string if it exists, or
+   *default* if it doesn't. *key* is a string. 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.
@@ -306,8 +306,8 @@ 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. Note that
+   Return the value of the environment variable *key* as bytes if it exists, or
+   *default* if it doesn't. *key* must be 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.