note breaking change in json_normalize retaining index

For context: #51542 & #57422

Author: JMBurley

doc/source/whatsnew/v3.0.0.rst

@@ -234,6 +234,7 @@ Other API changes
 - pickle and HDF (``.h5``) files created with Python 2 are no longer explicitly supported (:issue:`57387`)
 - pickled objects from pandas version less than ``1.0.0`` are no longer supported (:issue:`57155`)
 - when comparing the indexes in :func:`testing.assert_series_equal`, check_exact defaults to True if an :class:`Index` is of integer dtypes. (:issue:`57386`)
+- Passing a :class:`Series` input to :func:`json_normalize` will now retain the :class:`Series` :class:`Index`, previously output had a new 0:N index (:issue:`51452`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.deprecations: