Like nearly all software projects I've encountered, the WordPress one suffers from documentation debt. It's a situation that, in my opinion/experience, is likely to get worse before it gets better, unless something fundamental changes about the working practices.

Why? Well, because most architects, designers, and developers are busy and enthusiastic; getting on with their tasks. Documentation isn't usually part of their arena. Perhaps it should be. Imagine how much easier things would be if the documentation were done more in parallel with the development of other collateral, such as the source code. I think this can only happen if the developers become stakeholders in the documentation components.

It's probably a lot to ask, and possibly too much of a large or unwanted change to established procedures, but otherwise I think you'll forever be playing catch-up.