It's generally conceded that at least some documentation saves people time when "onboarding" to a new project. With that said, have there ever been any rigorous studies on how helpful various kinds of documentation tend to be? I'm particularly interested in whether there are studies that compare the relative effectiveness of different kinds of documentation in terms of time that it takes to onboard new developers to the project.
This is the only thing that I could find that is relevant for you I think:
I also worked at a firm that had done such a study inhouse. But on a more general scale of low effort documentation versus actual documentation.
The result of the inhouse study was that it increased the start up time for new hires drastically from one to two weeks time needed to get to a desired level of productivity to one week or less.