DOC: added whatsnew entry for PR#16047 addressing GH15520

Author: analyticalmonk

doc/source/whatsnew/v0.20.0.txt

@@ -1509,6 +1509,7 @@ Conversion
 - Bug in ``is_string_dtype``, ``is_timedelta64_ns_dtype``, and ``is_string_like_dtype`` in which an error was raised when ``None`` was passed in (:issue:`15941`)
 - Bug in the return type of ``pd.unique`` on a ``Categorical``, which was returning an ndarray and not a ``Categorical`` (:issue:`15903`)
 - Bug in ``Index.to_series()`` where the index was not copied (and so mutating later would change the original), (:issue:`15949`)
+- Bug in ``pandas_dtype`` where passing invalid dtype didn't raise an error. (:issue:`15520`)
 
 Indexing
 ^^^^^^^^