iio: accel: adxl372: Add additional trigger ABI docs

Document use of additional trigger supplied by driver.
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Link: https://lore.kernel.org/r/20200810093257.65929-3-alexandru.tachici@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-accel-adxl372

+What:		/sys/bus/iio/devices/triggerX/name = "adxl372-devX-peak"
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		The adxl372 accelerometer kernel module provides an additional trigger,
+		which sets the device in a mode in which it will record only the peak acceleration
+		sensed over the set period of time in the events sysfs.