viafb: add documentation for proc interface

This patch adds documentation for the new proc interface that allows
modification of the output device configuration. Should be stable and
useful enough now for daily use.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Andrew Morton <akpm@linux-foundation.org>