Correct more obsolete nbtree page split comments.

Correct more obsolete nbtree page split comments.

Commit 3f342839 corrected obsolete comments about buffer locks at the
main _bt_insert_parent() call site, but missed similar obsolete comments
above _bt_insert_parent() itself.  Both sets of comments were rendered
obsolete by commit 40dae7ec537, which made the nbtree page split
algorithm more robust.  Fix the comments that were missed the first time
around now.

In passing, refine a related _bt_insert_parent() comment about
re-finding the parent page to insert new downlink.