The four types of documentation
I love this graphic so much.
Great documentation covers all four parts of this graph.
The only thing I don't think it necessarily covers is examples. The best documentation in the world is still bl.ocks.org, which was a huge pool of example d3 usages that you could pick and choose the parts you wanted from. (Observable is good, but it's much more painful to pull example usages out of observable and into your code, so it's far less useful as documentation)