Backport PR #54896 on branch 2.1.x (DOC: updated docstring for deprecation of axis=1 in groupby) (#54912)

Backport PR #54896: DOC: updated docstring for deprecation of axis=1 in groupby

Co-authored-by: Rajat Subhra Mukherjee <[email protected]>

Author: meeseeksmachine

pandas/core/shared_docs.py

@@ -113,6 +113,12 @@
 axis : {0 or 'index', 1 or 'columns'}, default 0
     Split along rows (0) or columns (1). For `Series` this parameter
     is unused and defaults to 0.
+
+    .. deprecated:: 2.1.0
+
+        Will be removed and behave like axis=0 in a future version.
+        For ``axis=1``, do ``frame.T.groupby(...)`` instead.
+
 level : int, level name, or sequence of such, default None
     If the axis is a MultiIndex (hierarchical), group by a particular
     level or levels. Do not specify both ``by`` and ``level``.