DOC: cleanup whatsnew entry for na_action='ignore' (pandas-dev#52355)

* DOC: cleanup whatsnew entry for na_action='ignore'

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* update comment

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: topper-123

doc/source/whatsnew/v2.1.0.rst

@@ -21,8 +21,51 @@ enhancement1
 
 .. _whatsnew_210.enhancements.enhancement2:
 
-enhancement2
-^^^^^^^^^^^^
+``map(func, na_action="ignore")`` now works for all array types
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When given a callable, :meth:`Series.map` applies the callable to all elements of the :class:`Series`.
+Similarly, :meth:`DataFrame.applymap` applies the callable to all elements of the :class:`DataFrame`,
+while :meth:`Index.map` applies the callable to all elements of the :class:`Index`.
+
+Frequently, it is not desirable to apply the callable to nan-like values of the array and to avoid doing
+that, the ``map`` method could be called with ``na_action="ignore"``, i.e. ``ser.map(func, na_action="ignore")``.
+However, ``na_action="ignore"`` was not implemented for many ``ExtensionArray`` and ``Index`` types
+and ``na_action="ignore"`` did not work correctly for any ``ExtensionArray`` subclass except the nullable numeric ones (i.e. with dtype :class:`Int64` etc.).
+
+``na_action="ignore"`` now works for all array types (:issue:`52219`, :issue:`51645`, :issue:`51809`, :issue:`51936`, :issue:`52033`; :issue:`52096`).
+
+*Previous behavior*:
+
+.. code-block:: ipython
+
+    In [1]: ser = pd.Series(["a", "b", np.nan], dtype="category")
+    In [2]: ser.map(str.upper, na_action="ignore")
+    NotImplementedError
+    In [3]: df = pd.DataFrame(ser)
+    In [4]: df.applymap(str.upper, na_action="ignore")  # worked for DataFrame
+         0
+    0    A
+    1    B
+    2  NaN
+    In [5]: idx = pd.Index(ser)
+    In [6]: idx.map(str.upper, na_action="ignore")
+    TypeError: CategoricalIndex.map() got an unexpected keyword argument 'na_action'
+
+*New behavior*:
+
+.. ipython:: python
+
+    ser = pd.Series(["a", "b", np.nan], dtype="category")
+    ser.map(str.upper, na_action="ignore")
+    df = pd.DataFrame(ser)
+    df.applymap(str.upper, na_action="ignore")
+    idx = pd.Index(ser)
+    idx.map(str.upper, na_action="ignore")
+
+Also, note that :meth:`Categorical.map` implicitly has had its ``na_action`` set to ``"ignore"`` by default.
+This has been deprecated and will :meth:`Categorical.map` in the future change the default
+to ``na_action=None``, like for all the other array types.
 
 .. _whatsnew_210.enhancements.other:
 
@@ -34,6 +77,7 @@ Other enhancements
 - Implemented ``__pandas_priority__`` to allow custom types to take precedence over :class:`DataFrame`, :class:`Series`, :class:`Index`, or :class:`ExtensionArray` for arithmetic operations, :ref:`see the developer guide <extending.pandas_priority>` (:issue:`48347`)
 - :meth:`MultiIndex.sort_values` now supports ``na_position`` (:issue:`51612`)
 - :meth:`MultiIndex.sortlevel` and :meth:`Index.sortlevel` gained a new keyword ``na_position`` (:issue:`51612`)
+- :meth:`arrays.DatetimeArray.map`, :meth:`arrays.TimedeltaArray.map` and :meth:`arrays.PeriodArray.map` can now take a ``na_action`` argument (:issue:`51644`)
 - Improve error message when setting :class:`DataFrame` with wrong number of columns through :meth:`DataFrame.isetitem` (:issue:`51701`)
 - Let :meth:`DataFrame.to_feather` accept a non-default :class:`Index` and non-string column names (:issue:`51787`)
 - :class:`api.extensions.ExtensionArray` now has a :meth:`~api.extensions.ExtensionArray.map` method (:issue:`51809`)
@@ -42,7 +86,6 @@ Other enhancements
 - Improved error message when creating a DataFrame with empty data (0 rows), no index and an incorrect number of columns. (:issue:`52084`)
 - :meth:`DataFrame.applymap` now uses the :meth:`~api.extensions.ExtensionArray.map` method of underlying :class:`api.extensions.ExtensionArray` instances (:issue:`52219`)
 - :meth:`arrays.SparseArray.map` now supports ``na_action`` (:issue:`52096`).
-- :meth:`Categorical.map` and :meth:`CategoricalIndex.map` now have a ``na_action`` parameter (:issue:`44279`)
 - Add dtype of categories to ``repr`` information of :class:`CategoricalDtype` (:issue:`52179`)
 -
 
@@ -168,15 +211,15 @@ Categorical
 Datetimelike
 ^^^^^^^^^^^^
 - Bug in :meth:`Timestamp.round` with values close to the implementation bounds returning incorrect results instead of raising ``OutOfBoundsDatetime`` (:issue:`51494`)
-- :meth:`arrays.DatetimeArray.map` can now take a ``na_action`` argument. :meth:`DatetimeIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
+- :meth:`DatetimeIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
 - Bug in :meth:`arrays.DatetimeArray.map` and :meth:`DatetimeIndex.map`, where the supplied callable operated array-wise instead of element-wise (:issue:`51977`)
 -
 
 Timedelta
 ^^^^^^^^^
 - Bug in :meth:`Timedelta.round` with values close to the implementation bounds returning incorrect results instead of raising ``OutOfBoundsTimedelta`` (:issue:`51494`)
 - Bug in :class:`TimedeltaIndex` division or multiplication leading to ``.freq`` of "0 Days" instead of ``None`` (:issue:`51575`)
-- :meth:`arrays.TimedeltaArray.map` can now take a ``na_action`` argument. :meth:`TimedeltaIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
+- :meth:`TimedeltaIndex.map` with ``na_action="ignore"`` now works as expected (:issue:`51644`)
 - Bug in :meth:`arrays.TimedeltaArray.map` and :meth:`TimedeltaIndex.map`, where the supplied callable operated array-wise instead of element-wise (:issue:`51977`)
 -
 
@@ -232,7 +275,7 @@ I/O
 Period
 ^^^^^^
 - Bug in :class:`PeriodDtype` constructor failing to raise ``TypeError`` when no argument is passed or when ``None`` is passed (:issue:`27388`)
-- :meth:`arrays.PeriodArray.map` can now take a ``na_action`` argument. :meth:`PeriodIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
+- :meth:`PeriodIndex.map` with ``na_action="ignore"`` now works as expected (:issue:`51644`)
 - Bug in :class:`PeriodDtype` constructor raising ``ValueError`` instead of ``TypeError`` when an invalid type is passed (:issue:`51790`)
 - Bug in :meth:`arrays.PeriodArray.map` and :meth:`PeriodIndex.map`, where the supplied callable operated array-wise instead of element-wise (:issue:`51977`)
 -