staging:iio: Add documentation for IIO_EVENT_CODE

Document the different parameters of the IIO_EVENT_CODE macro and friends.

While we are at it standardise the name of channel type parameter.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>