buf.9: Sprinkle with mdoc macros

I did not bump the date here as the manual page looks more like a draft
and I'm not sure if it is actually up-to-date considering that it's
current Dd dates back to 1998.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52770

(cherry picked from commit fc7f5452c6ff1a9d01a9d8d0638da593ae4f080c)