Fix obsoleted comments for function prototypes

While doing the search in [1], I spotted several places where the
comments for the function prototypes are obsoleted. For instance,
btree_desc() and btree_identify() are now located in nbtdesc.c but the
comment in nbtxlog.h is still claiming they are in nbtxlog.c.

Fix these places with the attached. With high possibility there are
other places with this kind of obsoleted comments, but I don't know how
to find them all :-(.

Thanks
Richard

[1]
https://www.postgresql.org/message-id/CAMbWs489%2Bu6P_9qMjABsse0dNNBr36MA1SX5Ss7yZ7TD86mfKQ%40mail.gmail.com