C comment: add optimizer function reference

Reported-by: James Coleman

Discussion: https://postgr.es/m/CAAaqYe9F6uoMhAr+8rMLwvGzaKaSknPA0Wi3Ehtv8pbSYmJq-Q@mail.gmail.com

Backpatch-through: master

src/backend/optimizer/path/allpaths.c

@@ -3473,7 +3473,8 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
 			/*
 			 * Except for the topmost scan/join rel, consider gathering
 			 * partial paths.  We'll do the same for the topmost scan/join rel
-			 * once we know the final targetlist (see grouping_planner).
+			 * once we know the final targetlist (see grouping_planner's and
+			 * its call to apply_scanjoin_target_to_paths).
 			 */
 			if (!bms_equal(rel->relids, root->all_query_rels))
 				generate_useful_gather_paths(root, rel, false);