staging: iio: Documentation: Add missing sysfs docs for angle channel

The iio resolver drivers in staging use angle channels. This patch
add missing documentation for this type of channel.

As was discussed in [1], radians is chosen as the unit, to match the
unit of angular velocity.

[1] https://marc.info/?l=linux-driver-devel&m=152190078308330&w=2

Signed-off-by: David Veenstra <davidjulianveenstra@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>