Add missing period to HINT messages

Hi,

According to the error message style guide [1], "Detail and hint
messages: Use complete sentences, and end each with a period."

I found there is a small group of HINT messages not following that period rule.

PSA a patch that fixes the missing period. In passing, also fixed a
typo in a hint message /msut/must/.

~~~

On further inspection, lots of these affected messages don't even seem
grammatically correct to me.

I did not modify them, but here are examples of what I mean.

e.g.
"ALTER TABLE ... MERGE PARTITIONS can only merge partitions don't have
sub-partitions."
/don't have/that don't have/

e.g.
"ALTER TABLE ... SPLIT PARTITION can only split partitions don't have
sub-partitions."
/don't have/that don't have/

e.g.
"To split DEFAULT partition one of the new partition must be DEFAULT."
missing word? -- "To split the DEFAULT partition"
should be plural? -- "one of the new partitions"

e.g.
"%s require combined bounds of new partitions must exactly match the
bound of the split partition."
/require/requires/ ?
/match the bound/match the bounds/

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[1] https://www.postgresql.org/docs/devel/error-style-guide.html#ERROR-STYLE-GUIDE-GRAMMAR-PUNCTUATION

Kind Regards,
Peter Smith.
Fujitsu Australia