vt(4): Document kern.vt.fb.default_mode and kern.vt.fb.modes.*

Those tunables are used to set a specific mode in vt(4) instead of using
the default mode.

Differential Revision:	https://reviews.freebsd.org/D1098
Reviewed by:	ak@, emaste@, kwm@
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274050 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f