Documentation: ABI: IIO: add new convdelay documentation

Add new IIO "convdelay" documentation.

The ad7606 implements a phase calibation feature, in nanoseconds.
Being this a time delay, using the convdelay suffix.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250606-wip-bl-ad7606-calibration-v9-1-6e014a1f92a2@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>