#799369: document possible sys.platform values.

Doc/library/sys.rst

@@ -507,9 +507,26 @@ always available.
 
 .. data:: platform
 
-   This string contains a platform identifier, e.g. ``'sunos5'`` or ``'linux1'``.
-   This can be used to append platform-specific components to ``path``, for
-   instance.
+   This string contains a platform identifier that can be used to append
+   platform-specific components to :data:`sys.path`, for instance.
+
+   For Unix systems, this is the lowercased OS name as returned by ``uname -s``
+   with the first part of the version as returned by ``uname -r`` appended,
+   e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
+   For other systems, the values are:
+
+   ================ ===========================
+   System           :data:`platform` value
+   ================ ===========================
+   Windows          ``'win32'``
+   Windows/Cygwin   ``'cygwin'``
+   MacOS X          ``'darwin'``
+   MacOS 9          ``'mac'``
+   OS/2             ``'os2'``
+   OS/2 EMX         ``'os2emx'``
+   RiscOS           ``'riscos'``
+   AtheOS           ``'atheos'``
+   ================ ===========================
 
 
 .. data:: prefix