Explain the newfs naming convention

It might be unclear why newfs and newfs_msdos should cross-reference
each other. Add a note explaining it.

This is a follow-up to 74bd20769706041108a573601cf0b61c755bdc56.

Reported by: kib
Reviewed by: imp, kib, rpokala
MFC after: 3 days

(cherry picked from commit 5b9b65e92fb40703038cbcf61feb4616c42e0b6e)