Update obsolete comment.

Commit b52b7dc2, which moved code creating PartitionBoundInfo in
RelationBuildPartitionDesc() in partcache.c (relocated to partdesc.c
afterwards) to partbounds.c, should have updated this, but didn't.

Author: Etsuro Fujita
Reviewed-by: Alvaro Herrera
Backpatch-through: 12
Discussion: https://postgr.es/m/CAPmGK16Uxr%3DPatiGyaRwiQVLB7Y-GqbkK3AxRLVYzU0Czv%3DsEw%40mail.gmail.com

src/backend/partitioning/partbounds.c

@@ -360,9 +360,8 @@ create_list_bounds(PartitionBoundSpec **boundspecs, int nparts,
 			else
 			{
 				/*
-				 * Never put a null into the values array, flag instead for
-				 * the code further down below where we construct the actual
-				 * relcache struct.
+				 * Never put a null into the values array; save the index of
+				 * the partition that stores nulls, instead.
 				 */
 				if (null_index != -1)
 					elog(ERROR, "found null more than once");