[whatwg/streams] Clarify expected behaviour when breaking async iteration of teed ReadableStream (Issue #1343)

lukasIO created an issue (whatwg/streams#1343)

### What is the issue with the Streams Standard?

Is it intended by the spec that breaking async iteration of a teed streams halts the execution of the program?

```ts
const rs = new ReadableStream({
  start(controller) {
    let i = 0;
    const interval = setInterval(() => {
      controller.enqueue(i++);
    }, 100);

    // Clean up when canceled
    this._cancel = () => {
      clearInterval(interval);
      console.log('cancelled');
    };
  },
  cancel(reason) {
    console.log('cancel() called with', reason);
    this._cancel?.();
    return Promise.resolve();
  },
});

const [r1, r2] = rs.tee();

let i = 0;
(async () => {
  for await (const val of r1) {
    console.log('r1', val);
    if (i++ > 2) {
      console.log('breaking r1');
      break;
    }
  }
  console.log('finished r1 loop');
})();
```

Right now in all environments I tested in (browsers and across NodeJS versions) the last log I get is `"breaking r1"` - `finished r1 loop` never gets logged


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