Fix obsolete nbtree array advancement comment.

Fix obsolete nbtree array advancement comment.

Checking if another primitive scan is required after all once the next
leaf page was moved from _bt_checkkeys to its _bt_readpage caller by
commit 9a2e2a28.  Update a comment that incorrectly described the
recheck mechanism as something that takes place in _bt_checkkeys.

Also fix an older typo in related code comments.