pgsql: Add a comment to AdjustIntervalForTypmod to reduce chance of fut

Add a comment to AdjustIntervalForTypmod to reduce chance of future bugs.

It's not entirely evident how the logic here relates to the
interval_transform function, so let's clue people in that they need to
check that if the rules change.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d429ebe347a5f1404cc3c50234e1c93f8db2bb53

Modified Files
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src/backend/utils/adt/timestamp.c |    3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)