boot.config.5: Clarify that this controls legacy boot blocks

Reviewed by; ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52460

(cherry picked from commit e96359d111aaffa6bf0ea72a6f183eb3b91e6c72)
(cherry picked from commit de67d3cfc6537f6bfa8f2601da7af9c165d09075)