Correct code comments for PartitionedRelPruneInfo struct

The comments above the PartitionedRelPruneInfo struct incorrectly
document how subplan_map and subpart_map are indexed.  This seems to
have snuck in on 4e232364.

Author: David Rowley <david.rowley@2ndquadrant.com>

src/include/nodes/plannodes.h

@@ -1084,15 +1084,16 @@ typedef struct PartitionPruneInfo
  * PartitionedRelPruneInfo - Details required to allow the executor to prune
  * partitions for a single partitioned table.
  *
- * subplan_map[] and subpart_map[] are indexed by partition index (where
- * zero is the topmost partition, and non-leaf partitions must come before
- * their children).  For a leaf partition p, subplan_map[p] contains the
- * zero-based index of the partition's subplan in the parent plan's subplan
- * list; it is -1 if the partition is non-leaf or has been pruned.  For a
- * non-leaf partition p, subpart_map[p] contains the zero-based index of
- * that sub-partition's PartitionedRelPruneInfo in the hierarchy's
- * PartitionedRelPruneInfo list; it is -1 if the partition is a leaf or has
- * been pruned.  Note that subplan indexes are global across the parent plan
+ * subplan_map[] and subpart_map[] are indexed by partition index of the
+ * partitioned table referenced by 'rtindex', the partition index being the
+ * order that the partitions are defined in the table's PartitionDesc.  For a
+ * leaf partition p, subplan_map[p] contains the zero-based index of the
+ * partition's subplan in the parent plan's subplan list; it is -1 if the
+ * partition is non-leaf or has been pruned.  For a non-leaf partition p,
+ * subpart_map[p] contains the zero-based index of that sub-partition's
+ * PartitionedRelPruneInfo in the hierarchy's PartitionedRelPruneInfo list;
+ * it is -1 if the partition is a leaf or has been pruned.  Note that subplan
+ * indexes, as stored in 'subplan_map', are global across the parent plan
  * node, but partition indexes are valid only within a particular hierarchy.
  */
 typedef struct PartitionedRelPruneInfo