Trestle is the UI system that comes with Modula 3. Its programmers' manual is *excellent*. And, furthermore, it was machine-generated from the source code, which made it easy to keep the manual up-to-date as the code evolved.
And, yes, it still manages to be excellent.
There was a lot of thought put into it. There was a lot of thought put into writing the comments in the code so that they would yield comprehensible documentation, including a gradual (though quite technical) introduction to the subject matter.
Generating documentation from source code doesn't have to produce garbage, though it will if the programmer pays insufficient attention to the issue. And paying attention to the generated documentation during coding pays off in clean interface design, because a clean design makes documentation easier.