iio: Add ABI documentation for in_magn offset value

This patch adds ABI documentation entries for in_magn_offset.
There is one user, hid-sensor-magn-3d in drivers/iio/magnetometer.

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>