docs: ABI: unify /sys/class/leds/<led>/brightness documentation

This ABI is defined twice, one for normal leds and another
one for multicolor ones.

Ensure that just one definition is there at ABI.

Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1115022e7f172b19ee8610f2ad28cc2f0ca93592.1604042072.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>