#2476: document that %default feature is new in 2.4.

Doc/library/optparse.rst

@@ -534,7 +534,8 @@ help message:
   description "write output to FILE". This is a simple but effective way to make
   your help text a lot clearer and more useful for end users.
 
-* options that have a default value can include ``%default`` in the help
+.. versionadded:: 2.4
+   Options that have a default value can include ``%default`` in the help
    string---\ :mod:`optparse` will replace it with :func:`str` of the option's
    default value.  If an option has no default value (or the default value is
    ``None``), ``%default`` expands to ``none``.