add whatsnew note

Author: jorisvandenbossche

doc/source/whatsnew/v2.3.0.rst

@@ -53,6 +53,16 @@ These are bug fixes that might have notable behavior changes.
 notable_bug_fix1
 ^^^^^^^^^^^^^^^^
 
+.. _whatsnew_230.api_changes:
+
+API changes
+~~~~~~~~~~~
+
+- When enabling the ``future.infer_string`` option: Index set operations (like
+  union or intersection) will now ignore the dtype of an empty ``RangeIndex`` or
+  empty ``Index`` with object dtype when determining the dtype of the resulting
+  Index (:issue:`60797`)
+
 .. ---------------------------------------------------------------------------
 .. _whatsnew_230.deprecations:
 

doc/source/whatsnew/v3.0.0.rst

@@ -361,6 +361,9 @@ 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`)
+- Index set operations (like union or intersection) will now ignore the dtype of
+  an empty ``RangeIndex`` or empty ``Index`` with object dtype when determining
+  the dtype of the resulting Index (:issue:`60797`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.deprecations: