Documentation: ABI: testing: admv1014: add ABI docs

Add documentation for the use of the Digital Attenuator gain.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20220215081216.67706-3-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>