Documentation: iio: Clarify meaning of IIO_DISTANCE channel type

IIO_DISTANCE is used for two purposes: for pedometers to record the
distance covered by a walker, and to measure the distance to an object

IIO_DISTANCE is in meters while IIO_PROXIMITY is a unitless measure
indirectly proportional to distance (higher value relates to a closer
object)

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>