After #4120 was merged, here's a bit of polish on top.
- docs: Optimize JSDoc and comments.
- test: The existing 304 test is moved to the dedicated status code
block and extended with an `errorSpy` to also verify that no `error`
event is emitted.
- refactor: The success branch is moved before the error branch in
`fetch()`. This is more intuitive.
The bottom line is that this PR improves maintainability. For users,
this PR doesn't change anything.