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>