DOC: Fix "versionadded" for case_when (#58426)

Fix "versionadded" for case_when

Tag was on parameter instead of function itself.

Author: wjandrea

pandas/core/series.py

@@ -5359,6 +5359,8 @@ def case_when(
         """
         Replace values where the conditions are True.
 
+        .. versionadded:: 2.2.0
+
         Parameters
         ----------
         caselist : A list of tuples of conditions and expected replacements
@@ -5376,8 +5378,6 @@ def case_when(
             must not change the input Series
             (though pandas doesn`t check it).
 
-            .. versionadded:: 2.2.0
-
         Returns
         -------
         Series