staging:iio:Documentation sysfs-bus-iio add in and out prefixes

Also includes the inX -> in_voltageX and outX -> out_voltageX conversions.
To keep lines short quite a lot of supressing of directories was needed.
Signed-off-by: NJonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>