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

[emaste]

Summary

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
one or the other).  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