boot_i386.8: Mention this is legacy

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49704

(cherry picked from commit b8437cbae173308180fbaddc6a74bfb6f8c4410b)
(cherry picked from commit 96743bfb95431f396e71d35eb92258122233334f)