staging: iio: isl29018: add ABI documentation for infrared suppression

Add ABI documentation from the ISL29018 Data Sheet (FN6619.4, Oct 8,
2012) about the infrared suppression that can be controlled
with the proximity_on_chip_ambient_infrared_suppression sysfs attribute.
Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-light-isl29018

+What:		/sys/bus/iio/devices/iio:deviceX/proximity_on_chip_ambient_infrared_suppression
+Date:		January 2011
+KernelVersion:	2.6.37
+Contact:	linux-iio@vger.kernel.org
+Description:
+		From ISL29018 Data Sheet (FN6619.4, Oct 8, 2012) regarding the
+		infrared suppression:
+
+		Scheme 0, makes full n (4, 8, 12, 16) bits (unsigned) proximity
+		detection. The range of Scheme 0 proximity count is from 0 to
+		2^n. Logic 1 of this bit, Scheme 1, makes n-1 (3, 7, 11, 15)
+		bits (2's complementary) proximity_less_ambient detection. The
+		range of Scheme 1 proximity count is from -2^(n-1) to 2^(n-1).
+		The sign bit is extended for resolutions less than 16. While
+		Scheme 0 has wider dynamic range, Scheme 1 proximity detection
+		is less affected by the ambient IR noise variation.
+
+		0 Sensing IR from LED and ambient
+		1 Sensing IR from LED with ambient IR rejection