iio: document pitch, yaw, roll modifiers

Introduce ABI documentation for new modifiers used for reporting rotations
expressed as euler angles (i.e. yaw, pitch, roll).

It looks like we have some unit inconsistency along various IIO modifiers:
it seems that incli is in deg, angl is in radians and rot isn't documented,
but at least the adis16209 driver has rot in deg.

Here we use deg (so angl is the only one using radians).

Signed-off-by: Andrea Merello <andrea.merello@iit.it>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220907132205.28021-6-andrea.merello@iit.it


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