update test doc

Author: xiedeyantu

datafusion/sqllogictest/test_files/window.slt

@@ -2425,11 +2425,9 @@ SELECT c9, rn1 FROM (SELECT c9,
 145294611 96
 
 # test_c9_rn_ordering_alias_opposite_direction3
-# These test check for whether datafusion is aware of the ordering of the column generated by ROW_NUMBER() window function.
-# Physical plan should have a SortExec after BoundedWindowAggExec.
-# The reason is that ordering of the table after BoundedWindowAggExec can be described as rn1 ASC, and also c9 DESC.
-# However, the requirement is rn1 ASC, c9 ASC (lexicographical order). Hence existing ordering cannot satisfy requirement
-# (Requirement is finer than existing ordering)
+# These tests check whether DataFusion tracks the ordering of the column generated by ROW_NUMBER() window function.
+# The outer ORDER BY can be simplified by ordering equivalence, so the plan should not need an additional SortExec
+# beyond the one required to satisfy the window input order.
 query TT
 EXPLAIN SELECT c9, rn1 FROM (SELECT c9,
                        ROW_NUMBER() OVER(ORDER BY c9 DESC) as rn1