add follow-up PRs to whatsnew

doc/source/whatsnew/v0.19.0.txt

@@ -336,7 +336,7 @@ Other enhancements
 - Consistent with the Python API, ``pd.read_csv()`` will now interpret ``+inf`` as positive infinity (:issue:`13274`)
 - The ``DataFrame`` constructor will now respect key ordering if a list of ``OrderedDict`` objects are passed in (:issue:`13304`)
 - ``pd.read_html()`` has gained support for the ``decimal`` option (:issue:`12907`)
-- A function :func:`union_categorical` has been added for combining categoricals, see :ref:`Unioning Categoricals<categorical.union>` (:issue:`13361`)
+- A function :func:`union_categorical` has been added for combining categoricals, see :ref:`Unioning Categoricals<categorical.union>` (:issue:`13361`, :issue:`:13763`, issue:`13846')
 - ``Series`` has gained the properties ``.is_monotonic``, ``.is_monotonic_increasing``, ``.is_monotonic_decreasing``, similar to ``Index`` (:issue:`13336`)
 - ``DataFrame.to_sql()`` now allows a single value as the SQL type for all columns (:issue:`11886`).
 - ``Series.append`` now supports the ``ignore_index`` option (:issue:`13677`)