ACPI / video: update video_extension.txt for backlight control

The ACPI video driver has changed a lot, and it doesn't export
interfaces in /proc any more, so the documentation for it should
be updated.

This update focuses on ACPI video driver's backlight control.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>