iio:ABI docs: Fix issue around repeated definition of out_currentY_raw

This one is challenging as both the places this appears in specific drivers
are making 'unusual' uses of what looks like a simple output current
channel.
As a side note, this was particular bit of ABI occurs in other drivers
where the use is much more straight forward e.g. dac/ad5421

This patch attempts to make a best effort of adding it to the main docs but
retaining enough information. Both of these drivers probably need
specific documents being written to describe their unusual interfaces, but
those should be in the main documentation, not under Documentation/ABI.
That is a non trivial job so left for another time.

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/out_currentY_raw is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-light-lm3533-als:43  ./Documentation/ABI/testing/sysfs-bus-iio-health-afe440x:38

Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210117153816.696693-5-jic23@kernel.org