Remove references to sio(4) from NOTES

Also zs(4) and sab(4), which have similarly been removed.

Reviewed by: imp, jhb, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D33279

(cherry picked from commit 339ef8277350884079d7b8c46f21b5bdb63afac0)