Documentation: ABI: testing: rtq6056: Update ABI docs

Add documentation for the usage of voltage channel integration time.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1658242365-27797-4-git-send-email-u0084500@gmail.com


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