DOC: Add documentation regarding the bug (pandas-dev#60729)

Author: sanggon6107

doc/source/whatsnew/v3.0.0.rst

@@ -676,6 +676,7 @@ Interval
 
 Indexing
 ^^^^^^^^
+- Bug in :func:`Series.mask` unexpectedly filling pd.NA (:issue:`60729`)
 - Bug in :meth:`DataFrame.__getitem__` returning modified columns when called with ``slice`` in Python 3.12 (:issue:`57500`)
 - Bug in :meth:`DataFrame.from_records` throwing a ``ValueError`` when passed an empty list in ``index`` (:issue:`58594`)
 - Bug in :meth:`MultiIndex.insert` when a new value inserted to a datetime-like level gets cast to ``NaT`` and fails indexing (:issue:`60388`)