Documentation: ABI: document IIO in_proximity_nearlevel file

The vcnl4000 IIO driver introduced a new attribute
"in_proximity_nearlevel".  This adds it to the list of documented ABI
for sysfs-bus-iio.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-proximity

+What:		/sys/bus/iio/devices/iio:deviceX/in_proximity_nearlevel
+Date:		March 2020
+KernelVersion:	5.7
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Near level for proximity sensors. This is a single integer
+		value that tells user space when an object should be
+		considered close to the device. If the value read from the
+		sensor is above or equal to the value in this file an object
+		should typically be considered near.