#14692 Fix json docs to reflect changes in json.load

The behaviour of the parse_constant callback changed insofar that 'null',
'true', 'false' don't trigger its call anymore.

Patch by Serhiy Storchaka

Doc/library/json.rst

@@ -209,8 +209,8 @@ Basic Usage
    (e.g. :class:`float`).
 
    *parse_constant*, if specified, will be called with one of the following
-   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``, ``'null'``, ``'true'``,
-   ``'false'``.  This can be used to raise an exception if invalid JSON numbers
+   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.
+   This can be used to raise an exception if invalid JSON numbers
    are encountered.
 
    To use a custom :class:`JSONDecoder` subclass, specify it with the ``cls``