The two aren't quite interchangeable: the Node definition has a few
more fields, which means that URLs created naively with `new URL()`
aren't assignable to that type.
Because the doc comments for spec/js-api are written for
implementation authors, they aren't very useful as documentation for
end-users. This gives us a place to write user-facing documentation as
well as a test that verifies that the underlying type declarations are
kept in sync.
