Nobody reads your quickstart when things work. Everybody reads your error body when things break. That's your real documentation.
A good error tells you three things
What went wrong, in one human sentence.
Which field or input caused it.
What to do next, ideally with a link.
A 400 that just says 'Bad Request' is a locked door with no sign. A 400 with a field path and a hint is a door with a handle.
Machine-readable code, human-readable message, stable shape. That's the whole trick.