I opened the comments with an intention to write basically the same thing.

Higher level documentation - classes, packages, groups of packages - makes the project much more approachable. It answers the question "Here's a 3 levels deep hierarchy - where do I start, how are the pieces connected to each other?"

Documenting classes is fairly common, but only the public API. I'm not only interested in how to use the class, but also how does it work internally, what is the inner architecture.