iio: document inv_icm42600 driver private sysfs attributes

Add ABI documentation for inv_icm42600 private sysfs attributes.

Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20240605195949.766677-2-inv.git-commit@tdk.com


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