video/backlight: document sysfs interface

Date and KernelVersion may be wrong because the backlight
interface was introduced before git initial import.

Cc:Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>

Documentation/ABI/stable/sysfs-class-backlight

+What:		/sys/class/backlight/<backlight>/bl_power
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Control BACKLIGHT power, values are FB_BLANK_* from fb.h
+		 - FB_BLANK_UNBLANK (0)   : power on.
+		 - FB_BLANK_POWERDOWN (4) : power off
+Users:		HAL
+
+What:		/sys/class/backlight/<backlight>/brightness
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Control the brightness for this <backlight>. Values
+		are between 0 and max_brightness. This file will also
+		show the brightness level stored in the driver, which
+		may not be the actual brightness (see actual_brightness).
+Users:		HAL
+
+What:		/sys/class/backlight/<backlight>/actual_brightness
+Date:		March 2006
+KernelVersion:	2.6.17
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Show the actual brightness by querying the hardware.
+Users:		HAL
+
+What:		/sys/class/backlight/<backlight>/max_brightness
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Maximum brightness for <backlight>.
+Users:		HAL