Mention ordered datums in PartitionBoundInfoData comment

Hi,

Julien Rouhaund, who has proposed a patch for partition-wise ordering
mentioned to me offlist that the comments for PartitionBoundInfoData
do not mention the fact that the datums in datums array are ordered. I
think that's important to mention there. So here's patch to do that.

The comment I have added refers to the functions which order the
datums, since every partition kind has different method of ordering
datums and I think the prologue is not a suitable place to explain
that ordering. I have added a sentence for range and list partitioning
since the ordering is easier to explain in those cases. Also added a
sentence about canonical PartitionBoundInfoData without using that
word.

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Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company