iio: staging: fix device.txt documentation

- indio_dev->modes:

	Specify whether direct access and / or ring buffer access is supported.

- indio_dev->ring:

- indio_dev->buffer:

	An optional associated buffer.

- indio_dev->pollfunc:

	Poll function related elements. This controls what occurs when a trigger

- indio_dev->num_channels:

	How many channels are there?

Once these are set up, a call to iio_device_register(indio_dev),

Once these are set up, a call to iio_device_register(indio_dev)

will register the device with the iio core.

Worth noting here is that, if a ring buffer is to be used, it can be