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.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1213,6 +1213,32 @@ Description:
 		number or direction is not specified, applies to all channels of
 		this type.
 
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_en
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_rising_en
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_falling_en
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_en
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_rising_en
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_falling_en
+KernelVersion:	5.18
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Similar to in_accel_mag[_y][_rising|_falling]_en, but the event
+		value is relative to a reference magnitude. The reference magnitude
+		includes the graviational acceleration.
+
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_value
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_rising_value
+What:		/sys/.../iio:deviceX/events/in_accel_mag_referenced_falling_value
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_value
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_rising_value
+What:		/sys/.../iio:deviceX/events/in_accel_y_mag_referenced_falling_value
+KernelVersion:	5.18
+Contact:	linux-iio@vger.kernel.org
+Description:
+		The value to which the reference magnitude of the channel is
+		compared. If the axis is not specified, it applies to all channels
+		of this type.
+
 What:		/sys/.../events/in_steps_change_en
 KernelVersion:	4.0
 Contact:	linux-iio@vger.kernel.org