A small tweak to some comments for PartitionKeyData

While doing a bit of work on a partitioning patch I noticed that it's
not really that obvious that there's meant to be exactly 1 item in the
partexprs List for each zero-valued partattrs element. Some incorrect
code using these fields was the cause of CVE-2018-1052, so I think
it's worthwhile to mention how they should be used in the comments.

Patch attached.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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