GetTupleForTrigger could use a header comment

GetTupleForTrigger seems to be a fairly important part of the trigger
machinery, yet it has no header comment.  I think it should, although
I don't understand it well enough at this point to write one.  Right
now, one is left to guess at the overall purpose of the function and
the meanings of the arguments.  For example, it does something very
different based on whether newSlot != NULL, with zero explanation as
to the underlying reasons or theory of operation.  Whee!

-- 
Robert Haas
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