bpo-39383: Mention Darwin as a potential value for platform.system() (GH-18054)

(cherry picked from commit 8d57a41)

Co-authored-by: Peter Bittner <[email protected]>

Author: miss-islington

Doc/library/platform.rst

@@ -140,8 +140,8 @@ Cross Platform
 
 .. function:: system()
 
-   Returns the system/OS name, e.g. ``'Linux'``, ``'Windows'``, or ``'Java'``. An
-   empty string is returned if the value cannot be determined.
+   Returns the system/OS name, such as ``'Linux'``, ``'Darwin'``, ``'Java'``,
+   ``'Windows'``. An empty string is returned if the value cannot be determined.
 
 
 .. function:: system_alias(system, release, version)
@@ -281,4 +281,3 @@ Unix Platforms
    using :program:`gcc`.
 
    The file is read and scanned in chunks of *chunksize* bytes.
-