Well, documentation is scattered around. You need board schematic, which can be found on orangepi.org. Then you will probably want to see Allwinner H6 user manual, which can be found on linux-sunxi.org. Last but not least, you'll want to see CEC registers description. Unfortunately, you have to get creative here and find leaked DW HDMI documentation.
In any case, none of that will tell you anything about Pulse Eight adapter. You have to approach that one from Kodi perspective. Pulse Eight libcec library was first attempt at trying to make standard CEC library which can be used by applications. At that time, there were absolutely no standard kernel interface for CEC, so every device and SoC manufacturer did something hackish and incompatible with other solutions, even if same underlying HW was used. As you can imagine, Kodi used that to have any hope of supporting CEC on multiple devices. Later on, when Linux kernel finally got common CEC interface, Pulse Eight libcec library was extended to support this interface. Since Allwinner SoCs expose CEC functionality over this common kernel CEC interface, Pulse Eight library naturally also supports Allwinner SoCs (when using mainline kernel).
There is idea to get rid of Pulse Eight libcec dependency and use Linux CEC interface directly, but it's a lot of work that currently nobody wants to do.