bpo-35344: platform.platform() uses mac_ver() on macOS (GH-10780)

On macOS, platform.platform() now uses mac_ver(), if it returns a
non-empty release string, to get the macOS version rather than darwin
version.
diff --git a/Doc/library/platform.rst b/Doc/library/platform.rst
index 7ac4b02..60c6089 100644
--- a/Doc/library/platform.rst
+++ b/Doc/library/platform.rst
@@ -79,6 +79,11 @@
    Setting *terse* to true causes the function to return only the absolute minimum
    information needed to identify the platform.
 
+   .. versionchanged:: 3.8
+      On macOS, the function now uses :func:`mac_ver`, if it returns a
+      non-empty release string, to get the macOS version rather than the darwin
+      version.
+
 
 .. function:: processor()