vt(4): add note about sc/UEFI incompatibility

syscons is not compatible with UEFI boot. This is noted in syscons(4),
but not mentioned in vt(4) where the kern.vty tunable (used to select vt
or sc) is documented. Add a note so that if someone reads vt(4) but not
sc(4) they are not surprised by having no usable console.

PR: 276206
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45357