pgsql: Update obsolete row compare preprocessing comments.

Update obsolete row compare preprocessing comments. Restore nbtree preprocessing comments describing how we mark nbtree row compare members required to how they were prior to 2016 bugfix commit a298a1e0. Oversight in commit bd3f59fd, which made nbtree preprocessing revert to the original 2006 rul

pgsql: Update obsolete row compare preprocessing comments.

Update obsolete row compare preprocessing comments. Restore nbtree preprocessing comments describing how we mark nbtree row compare members required to how they were prior to 2016 bugfix commit a298a1e0. Oversight in commit bd3f59fd, which made nbtree preprocessing revert to the original 2006 rul