add whatsnew entry for pandas-dev#42365

Author: neelmraman

doc/source/whatsnew/v1.4.0.rst

@@ -159,7 +159,7 @@ Interval
 Indexing
 ^^^^^^^^
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`DatetimeIndex` when passing a string, the return type depended on whether the index was monotonic (:issue:`24892`)
--
+- Bug in :meth:`DataFrame.truncate` and :meth:`Series.truncate` when the object's Index has a length greater than one but only one unique value (:issue:`42365`)
 
 Missing
 ^^^^^^^