[scottlamb]

I think you need both. You're not going to get a documentation team to successfully write API docs for every nook and cranny of the codebase if those nook and crannies' respective coders are uninterested. But you're also not going to get a bunch of coders whose main job is extending the language to write the Rust Book on the side. To me good documentation means that both those things exist.

[dan_quixote]

You absolutely need both for quality, public-facing docs. The process of explaining the logic to your docs expert is just invaluable. Without the docs expert, the engineer(s) is too in-the-weeds to meet the audience's needs thoroughly. Without the engineer(s), the docs expert has too steep a hill to climb to understand the intent and edge cases.