Staging: IIO: Direct digital synthesis abi documentation

Changes since RFC/v1:
IIO: Apply list review feedback:

Apply list review feedback:
	Restructure documentation according to list feedback.
	Rename attributes to fit IIO convention used in other drivers.
	Fix typos.
	Provide ddsX_out_enable as opposed to ddsX_out_disable

Changes since v2:
IIO: sysfs-bus-iio-dds: Apply more list review feedback

	Fix mixture of X and [n] notation for indices.
	Fix some wording in the descriptions.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>