docs: iio: describe inactivity and free-fall detection on the ADXL345

docs: iio: describe inactivity and free-fall detection on the ADXL345

Describe the inactivity detection additionally using the free-fall
register. Due to the controversial discussions on the mailing list, this
section of the documentation will be committed separately to allow for a
more focused and detailed elaboration of the topic.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250727210014.27766-8-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>