add issue number

Author: jorisvandenbossche

doc/source/whatsnew/v2.3.1.rst

@@ -23,7 +23,7 @@ become the default string dtype in pandas 3.0. See
 Comparisons between different string dtypes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-In previous versions, comparing :class:`Series` of different string dtypes (e.g. ``pd.StringDtype("pyarrow", na_value=pd.NA)`` against ``pd.StringDtype("python", na_value=np.nan)``) would result in inconsistent resulting dtype or incorrectly raise. pandas will now use the hierarchy
+In previous versions, comparing :class:`Series` of different string dtypes (e.g. ``pd.StringDtype("pyarrow", na_value=pd.NA)`` against ``pd.StringDtype("python", na_value=np.nan)``) would result in inconsistent resulting dtype or incorrectly raise (:issue:`60639`). pandas will now use the hierarchy
 
     object < (python, NaN) < (pyarrow, NaN) < (python, NA) < (pyarrow, NA)