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>