A small tweak to some comments for PartitionKeyData

It was 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
it's worthwhile to mention how they should be used in the comments.

Author: David Rowley <david.rowley@2ndquadrant.com>
