docs: bhyve: document ignoring unknown MSRs

Ignoring unknown MSRs using <features> element
<msrs unknown='ignore'/> was supported for quite some already,
so add documentation for it for completeness of flags coverage,
as some guests can be extra picky about flags passed to bhyve,
and it's useful to know how to control those.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>