iio: ABI: document mag_referenced

iio: ABI: document mag_referenced

Some accelerometers that support activity and inactivity
events also support a referenced mode, in which the
gravitational acceleration is taken as a point of
reference before comparing the acceleration to the
specified activity and inactivity magnitude.

For example, in the case of the ADXL367, for activity
detection, the formula is:

abs(acceleration - reference) > magnitude

Add a new event type that makes this behavior clear.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20220214073810.781016-3-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>