Backport PR #45587: DOC: append deprecation (#45942)

Co-authored-by: gesoos <66533786+gesoos@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v1.4.0.rst

@@ -637,11 +637,11 @@ will continue to return :class:`Int64Index`, :class:`UInt64Index` and
 
 .. _whatsnew_140.deprecations.frame_series_append:
 
-Deprecated Frame.append and Series.append
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Deprecated DataFrame.append and Series.append
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 :meth:`DataFrame.append` and :meth:`Series.append` have been deprecated and will
-be removed in Pandas 2.0. Use :func:`pandas.concat` instead (:issue:`35407`).
+be removed in a future version. Use :func:`pandas.concat` instead (:issue:`35407`).
 
 *Deprecated syntax*
 

pandas/core/frame.py

@@ -8930,6 +8930,10 @@ def append(
         """
         Append rows of `other` to the end of caller, returning a new object.
 
+        .. deprecated:: 1.4.0
+            Use :func:`concat` instead. For further details see
+            :ref:`whatsnew_140.deprecations.frame_series_append`
+
         Columns in `other` that are not in the caller are added as new columns.
 
         Parameters

pandas/core/series.py

@@ -2853,6 +2853,10 @@ def append(
         """
         Concatenate two or more Series.
 
+        .. deprecated:: 1.4.0
+            Use :func:`concat` instead. For further details see
+            :ref:`whatsnew_140.deprecations.frame_series_append`
+
         Parameters
         ----------
         to_append : Series or list/tuple of Series