updated shift docstring to mark Series behavior

Author: sharkipelago

pandas/core/generic.py

@@ -10216,6 +10216,7 @@ def shift(
         suffix : str, optional
             If str and periods is an iterable, this is added after the column
             name and before the shift value for each shifted column name.
+            For `Series` this parameter is unused and defaults to `None`.
 
         Returns
         -------