Documentation of iio_read_channel_processed return value

[Luca Weiss]

Hi all,

I've noticed that the docstring in include/linux/iio/consumer.h for
iio_read_channel_processed (and likely some other functions in there) is
wrong because it's saying "Returns an error code or 0." while often it's
returning e.g. IIO_VAL_INT. In some code paths it's actually returning 0
but it seems in standard cases it's not.

Could the docstring please be updated so this is clear to users of this
function? I don't understand IIO well enough to confidently propose
changes there myself.

Thanks and kind regards
Luca