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.com


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