Console is utf-8 by default

The console is now by default in UTF-8 mode.  Fix the documentation on
the default value, so that we can explain behaviour that otherwise
causes bug-reports like this:

	http://bugzilla.kernel.org/show_bug.cgi?id=9319

Also add the needed "vt." prefix, so that the boot-time config options
to switch back to the legacy 8-bit mode is actually documented
correctly.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>