Clean up API docs for stable modules

Certain modules, such as for wiring diagrams, are approaching a reasonable degree of API stability. It would be good to improve the API docs. This means following the Julia [docstring conventions](https://docs.julialang.org/en/v1/manual/documentation/index.html) and using [Documenter.jl features](https://juliadocs.github.io/Documenter.jl/stable/man/syntax/) such as `@ref` and `@id`, which I have not been consistently doing.