But as always, the documentation is ad hoc. There are many different frameworks one might use for your server side implementation. Each of them has a different approach to documentation. Just choosing between the different frameworks (in this case that works with node.js) is itself a chore and a half.
I presume that the people involved in all these technologies and frameworks are not lazy nor stupid. I suspect that there is a combination of things going on here. They include such things as (a) being not particularly talented at writing documentation nor enjoying the process, (b) not realizing that such documentation is necessary, (c) balancing the needs of this project with other responsibilities, (d) relying on someone else to do it, and (e) actually believing the groupsource myth that says that other people will write it for you.
[Addendum. As time goes by, I penetrate more of the mysteries and it is not too bad. In fact, it may even be reasonable. But Jesus, they really don't try to make it easy for you.]