docs: iio: add documentation for device buffers

Add documentation for IIO device buffers describing buffer
attributes and how data is structured in buffers using
scan elements.

Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Link: https://lore.kernel.org/r/20240221085848.991413-3-ramona.gradinariu@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>