led: document sysfs interface

Also fix Documentation/led-class.txt, the acceptable
range of values for brightness is 0-max_brightness, not 0-255.

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/testing/sysfs-class-led

@@ -0,0 +1,28 @@
+What:		/sys/class/leds/<led>/brightness
+Date:		March 2006
+KernelVersion:	2.6.17
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Set the brightness of the LED. Most LEDs don't
+		have hardware brightness support so will just be turned on for
+		non-zero brightness settings. The value is between 0 and
+		/sys/class/leds/<led>/max_brightness.
+
+What:		/sys/class/leds/<led>/max_brightness
+Date:		March 2006
+KernelVersion:	2.6.17
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Maximum brightness level for this led, default is 255 (LED_FULL).
+
+What:		/sys/class/leds/<led>/trigger
+Date:		March 2006
+KernelVersion:	2.6.17
+Contact:	Richard Purdie <rpurdie@rpsys.net>
+Description:
+		Set the trigger for this LED. A trigger is a kernel based source
+		of led events.
+		You can change triggers in a similar manner to the way an IO
+		scheduler is chosen. Trigger specific parameters can appear in
+		/sys/class/leds/<led> once a given trigger is selected.
+