Backport PR pandas-dev#52453: DOC: update pivot docs for version pandas 2.x

Author: srkds

pandas/core/frame.py

@@ -8285,8 +8285,7 @@ def groupby(
                Also accept list of columns names.
 
         index : str or object or a list of str, optional
-            Column to use to make new frame's index. If None, uses
-            existing index.
+            Column to use to make new frame's index. If not given, uses existing index.
 
             .. versionchanged:: 1.1.0
                Also accept list of index names.