DOC: More 0.22.0 updates (pandas-dev#19002)

[ci skip]

(cherry picked from commit d7008b8)

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -372,6 +372,12 @@ Additionally, support has been dropped for Python 3.4 (:issue:`15251`).
 Sum/Prod of all-NaN or empty Series/DataFrames is now consistently NaN
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+.. note::
+
+   The changes described here have been partially reverted. See
+   the :ref:`v0.22.0 Whatsnew <whatsnew_0220>` for more.
+
+
 The behavior of ``sum`` and ``prod`` on all-NaN Series/DataFrames no longer depends on
 whether `bottleneck <http://berkeleyanalytics.com/bottleneck>`__ is installed, and return value of ``sum`` and ``prod`` on an empty Series has changed (:issue:`9422`, :issue:`15507`).
 

doc/source/whatsnew/v0.22.0.txt

@@ -218,3 +218,26 @@ returns ``0``.
 
 The default behavior of ``min_periods=None``, implying that ``min_periods``
 equals the window size, is unchanged.
+
+Compatibility
+~~~~~~~~~~~~~
+
+If you maintain a library that should work across pandas versions, it
+may be easiest to exclude pandas 0.21 from your requirements. Otherwise, all your
+``sum()`` calls would need to check if the ``Series`` is empty before summing.
+
+With setuptools, in your ``setup.py`` use::
+
+    install_requires=['pandas!=0.21.*', ...]
+
+With conda, use
+
+.. code-block:: yaml
+
+    requirements:
+      run:
+        - pandas !=0.21.0,!=0.21.1
+
+Note that the inconsistency in the return value for all-*NA* series is still
+there for pandas 0.20.3 and earlier. Avoiding pandas 0.21 will only help with
+the empty case.

pandas/core/generic.py

@@ -7133,8 +7133,7 @@ def _doc_parms(cls):
 ----------
 axis : %(axis_descr)s
 skipna : boolean, default True
-    Exclude NA/null values. If an entire row/column is NA or empty, the result
-    will be NA
+    Exclude NA/null values when computing the result.
 level : int or level name, default None
     If the axis is a MultiIndex (hierarchical), count along a
     particular level, collapsing into a %(name1)s
@@ -7268,7 +7267,7 @@ def _doc_parms(cls):
     The required number of valid values to perform the operation. If fewer than
     ``min_count`` non-NA values are present the result will be NA.
 
-    .. versionadded :: 0.21.2
+    .. versionadded :: 0.22.0
 
        Added with the default being 1. This means the sum or product
        of an all-NA or empty series is ``NaN``.