docs: iio: ad7380: add alert support

Add a section for alert support, explaining how user can use iio events
attributes to enable alert and set thresholds.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20250108-ad7380-add-alert-support-v4-5-1751802471ba@baylibre.com


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