iio:Documentation:ABI Add missing elements as used by the adi,ad7150

Main additions are around thresh_adaptive.  This has been supported
by the core of IIO for a long time, but no driver that uses it has
previously graduated from staging, hence we are missing Docs.

Otherwise, just new entries in existing lists.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210314181511.531414-22-jic23@kernel.org