Add whatsnew

Author: Matt Roeschke

doc/source/whatsnew/v0.24.0.txt

@@ -182,6 +182,7 @@ Other Enhancements
 - :func:`to_timedelta` now supports iso-formated timedelta strings (:issue:`21877`)
 - :class:`Series` and :class:`DataFrame` now support :class:`Iterable` in constructor (:issue:`2193`)
 - :class:`DatetimeIndex` gained :attr:`DatetimeIndex.timetz` attribute. Returns local time with timezone information. (:issue:`21358`)
+- :meth:`round`, :meth:`ceil`, and meth:`floor` for :class:`DatetimeIndex` and :class:`Timestamp` now support an ``ambiguous`` argument for handling datetimes that are rounded to ambiguous times (:issue:`18946`)
 - :class:`Resampler` now is iterable like :class:`GroupBy` (:issue:`15314`).
 - :ref:`Series.resample` and :ref:`DataFrame.resample` have gained the :meth:`Resampler.quantile` (:issue:`15023`).