ACPI: video: update doc for acpi_video_bus_DOS()

Commit efaa14c:

"Starting from win8, MS backlight control driver will set bit 2 of the
parameter of control method _DOS, to inform firmware it should not
perform any automatic brightness changes. This mostly affects hotkey
notification deliver - if we do not set this bit, on hotkey press,
firmware may choose to adjust brightness level instead of sending out
notification and doing nothing."

win7:
https://download.microsoft.com/download/9/c/5/9c5b2167-8017-4bae-9fde-d599bac8184a/BrightnessCtrl.docx

"To avoid problems that might occur if both the system firmware and
the monitor driver control the brightness of the display, the display
miniport driver should set bit 2 of the argument to the _DOS method.
Setting this bit notifies the system firmware that it should not
perform any automatic display brightness changes. The WDDM
driver must set this particular bit because it controls the _DOS
method. The other bits in the _DOS method control the behavior of
the firmware in response to the display switch hot keys."

win8:
http://read.pudn.com/downloads193/doc/907411/Brightness.doc

Signed-off-by: Kacper Piwiński <cosiekvfj@o2.pl>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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