Documenting for developers ​
Cf the documentation introduction, we are referring to knowledge targeted at developers and ever-changing (at least).
Developers are interested in all the layers, from the helicopter view to the most micro detail.
The macro part is already documented via the documentation for business, so remain the details.
High-level knowledge ​
To understand how things interconnect, use tools like dependency-cruiser or Madge. On one hand they let you forbid certain internal dependencies – which is awesome –, and on the other hand they can draw graphs of your modules. How awesome is that?
Mid-level knowledge – Who is using that function outside my domain? ​
Use the "find references" IDE feature.
Low-level ​
Use JSDoc @example
comments. And certify them using generate-jsdoc-example-tests.