docs: ABI: sysfs-class-backlight: unify ABI documentation

Both adp8860 and adp8870 define some extensions to the
backlight class. This causes warnings:

	Warning: /sys/class/backlight/<backlight>/ambient_light_level is defined 2 times:  /sys/class/backlight/<backlight>/ambient_light_level:8  /sys/class/backlight/<backlight>/ambient_light_level:30
	Warning: /sys/class/backlight/<backlight>/ambient_light_zone is defined 2 times:  /sys/class/backlight/<backlight>/ambient_light_zone:18  /sys/class/backlight/<backlight>/ambient_light_zone:40

As ABI definitions shouldn't be duplicated.

Unfortunately, the ABI is dependent on the specific device
features. As such, ambient_light_level range is somewhat
different among the supported devices.

The ambient_light_zone is even worse: the meanings of each
preset are different, and there's no ABI to retrieve
the supported types nor their meanins. Unfortunately,
it is too late to fix it without causing regressions,
as this has been used since Kernel v2.6.35.

Rewrite those ABI documentation using the current documentation
as a reference, and double-checking at the datasheets:

	https://www.analog.com/media/en/technical-documentation/data-sheets/ADP8870.pdf
	https://www.analog.com/media/en/technical-documentation/data-sheets/ADP8860.pdf

in order to properly document the differences between those two
drivers.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/342195ad5a819d9bcfcebc133c77ab69b4211672.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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