I totally agree with you, but you have to keep the difference between the projects in mind.
The documentation stands and falls with people actually contributing to it. Many successful Open Source projects have maintainers just dedicated to writing documentation. I don’t think cocos2d-x has such a system in place.
If you look at the php documentation notes. you will notice, that there are 20-30 documentation maintainers and coordinators just doing and contributing to one thing: documentation.
There are projects, where some maintainers are paid for the work they do or even have hundreds of maintainers. I don’t think cocos2d-x have that much resources or even contributors/maintainers.