loader.kboot: smbios: Add a comment about v3 entry point being favored

Note that the behavior here is consistent with BIOS and EFI boot.

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49293

(cherry picked from commit 0f492f79084f2f88c9d5ade92f900e26cc47a7d0)