Documentation:ABI:testing:admv1013: add ABI docs

Add documentation for the use of the Local Oscillator Feedthrough Offset
calibration.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20211221112206.97066-3-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>