iio: document linear acceleration modifiers

Introduce ABI documentation for new IIO modifiers used for reporting
"linear acceleration" measures.

Signed-off-by: Andrea Merello <andrea.merello@iit.it>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220907132205.28021-3-andrea.merello@iit.it


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