Add BTOPTIONS_PROC comments to nbtree.h.

Add BTOPTIONS_PROC comments to nbtree.h.

Add comments explaining the purpose of B-Tree support function 5 to
nbtree.h for consistency (all other support functions were already
described by nearby comments).

This fixes what was arguably an oversight in commit 911e702077, or in
follow-up doc commit 15cb2bd2 (which documented support function 5 in
btree.sgml, but neglected to add anything to nbtree.h).