bpo-15450: Allow subclassing of dircmp (GH-23424) (#23424)

Co-authored-by: Chris Jerdonek <chris.jerdonek@gmail.com>
diff --git a/Doc/library/filecmp.rst b/Doc/library/filecmp.rst
index 31b9b4a..c60603b 100644
--- a/Doc/library/filecmp.rst
+++ b/Doc/library/filecmp.rst
@@ -173,7 +173,13 @@
    .. attribute:: subdirs
 
       A dictionary mapping names in :attr:`common_dirs` to :class:`dircmp`
-      objects.
+      instances (or MyDirCmp instances if this instance is of type MyDirCmp, a
+      subclass of :class:`dircmp`).
+
+      .. versionchanged:: 3.10
+         Previously entries were always :class:`dircmp` instances. Now entries
+         are the same type as *self*, if *self* is a subclass of
+         :class:`dircmp`.
 
 .. attribute:: DEFAULT_IGNORES