gh-141510: Document frozendict support in json (gh-155929)

Author: fregataa

Doc/library/json.rst

@@ -486,7 +486,7 @@ Encoders and Decoders
    +----------------------------------------+---------------+
    | Python                                 | JSON          |
    +========================================+===============+
-   | dict                                   | object        |
+   | dict, frozendict                       | object        |
    +----------------------------------------+---------------+
    | list, tuple                            | array         |
    +----------------------------------------+---------------+
@@ -504,6 +504,9 @@ Encoders and Decoders
    .. versionchanged:: 3.4
       Added support for int- and float-derived Enum classes.
 
+   .. versionchanged:: 3.15
+      Added support for :class:`frozendict`.
+
    To extend this to recognize other objects, subclass and implement a
    :meth:`~JSONEncoder.default` method with another method that returns a serializable object
    for ``o`` if possible, otherwise it should call the superclass implementation