Improve comment about PartitionBoundInfoData.

Ashutosh Bapat, per discussion with Julien Rouhaund, who also
reviewed this patch.

Discussion: http://postgr.es/m/CAFjFpReBR3ftK9C23LLCZY_TDXhhjB_dgE-L9+mfTnA=gkvdvQ@mail.gmail.com

src/backend/catalog/partition.c

@@ -72,6 +72,13 @@
  * of datum-tuples with 2 datums, modulus and remainder, corresponding to a
  * given partition.
  *
+ * The datums in datums array are arranged in increasing order as defined by
+ * functions qsort_partition_rbound_cmp(), qsort_partition_list_value_cmp() and
+ * qsort_partition_hbound_cmp() for range, list and hash partitioned tables
+ * respectively. For range and list partitions this simply means that the
+ * datums in the datums array are arranged in increasing order as defined by
+ * the partition key's operator classes and collations.
+ *
  * In the case of list partitioning, the indexes array stores one entry for
  * every datum, which is the index of the partition that accepts a given datum.
  * In case of range partitioning, it stores one entry per distinct range