DOC: Add whatsnew note

Author: Licht-T

doc/source/whatsnew/v0.23.0.txt

@@ -1356,6 +1356,7 @@ Reshaping
 - Bug in :meth:`DataFrame.astype` where column metadata is lost when converting to categorical or a dictionary of dtypes (:issue:`19920`)
 - Bug in :func:`cut` and :func:`qcut` where timezone information was dropped (:issue:`19872`)
 - Bug in :class:`Series` constructor with a ``dtype=str``, previously raised in some cases (:issue:`19853`)
+- Bug in :func:`isna`, which cannot handle ambiguous typed lists (:issue:`20675`)
 
 Other
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