Fix outdated comments

A few fields in ResultRelInfo are now also used for MERGE.  Update the
comments to mention that.

Reported-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxH8-NvFhLcSZZTTW+1M9AfS4+SOTKmyPG7ZhzNvN=+NkA@mail.gmail.com:wq

Author: david-rowley

src/include/nodes/execnodes.h

@@ -465,8 +465,8 @@ typedef struct ResultRelInfo
 	IndexInfo **ri_IndexRelationInfo;
 
 	/*
-	 * For UPDATE/DELETE result relations, the attribute number of the row
-	 * identity junk attribute in the source plan's output tuples
+	 * For UPDATE/DELETE/MERGE result relations, the attribute number of the
+	 * row identity junk attribute in the source plan's output tuples
 	 */
 	AttrNumber	ri_RowIdAttNo;
 
@@ -524,7 +524,9 @@ typedef struct ResultRelInfo
 	/* array of constraint-checking expr states */
 	ExprState **ri_ConstraintExprs;
 
-	/* arrays of stored generated columns expr states, for INSERT and UPDATE */
+	/*
+	 * Arrays of stored generated columns ExprStates for INSERT/UPDATE/MERGE.
+	 */
 	ExprState **ri_GeneratedExprsI;
 	ExprState **ri_GeneratedExprsU;