filemon: Document how to build into the kernel.

MFC after: 3 days
Reviewed by: olce, imp
Differential Revision: https://reviews.freebsd.org/D46184

(cherry picked from commit 057453ffdf57bdfcbadb5d1834d425812e01ff43)

filemon: Not an option.

MFC after: 3 days

(cherry picked from commit b4e4512d4669267fb220b7c1a2844c506673a3ce)