ACPI / video: add description for brightness_switch_enabled

Add description for video module's parameter brightness_switch_enabled
into kernel-parameters.txt.
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/kernel-parameters.txt

@@ -3229,6 +3229,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	video=		[FB] Frame buffer configuration
 			See Documentation/fb/modedb.txt.
 
+	video.brightness_switch_enabled= [0,1]
+			If set to 1, on receiving an ACPI notify event
+			generated by hotkey, video driver will adjust brightness
+			level and then send out the event to user space through
+			the allocated input device; If set to 0, video driver
+			will only send out the event without touching backlight
+			brightness level.
+			default: 1
+
 	virtio_mmio.device=
 			[VMMIO] Memory mapped virtio (platform) device.