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 rules, but neglected to revert these comments.

Backpatch-through: 18