update docstring

Author: sfc-gh-vbudati

tests/ast/decoder.py

@@ -10,6 +10,7 @@
 
 from pandas import DataFrame as PandasDataFrame
 
+from snowflake.snowpark.table import WhenMatchedClause, WhenNotMatchedClause
 from snowflake.snowpark.window import Window, WindowRelativePosition
 import snowflake.snowpark._internal.proto.generated.ast_pb2 as proto
 
@@ -536,14 +537,21 @@ def decode_join_type(self, join_type: proto.SpJoinType) -> str:
                     "Unknown join type: %s" % join_type.WhichOneof("variant")
                 )
 
-    def decode_matched_clause(self, matched_clause: proto.SpMatchedClause):
+    def decode_matched_clause(
+        self, matched_clause: proto.SpMatchedClause
+    ) -> Union[WhenMatchedClause, WhenNotMatchedClause]:
         """
         Decode a matched clause expression to get the clause.
 
         Parameters
         ----------
         matched_clause : proto.SpMatchedClause
             The expression to decode.
+
+        Returns
+        -------
+        WhenMatchedClause or WhenNotMatchedClause
+            The decoded clause.
         """
         match matched_clause.WhichOneof("variant"):
             case "sp_merge_delete_when_matched_clause":