Re: [whatwg/streams] Clarify that a pull source doesn't need to wait in pull() (#1014)

I don't think this part is correct:
> When a source blocks inside `pull()`, it is not possible to cancel the ReadableStream until the promise returned by `pull()` settles.

As far as I know, we don't actually wait for the pending `pull()` to settle before we call `cancel()` on the source.
```javascript
var rs = new ReadableStream({
  async pull(c) {
    console.log("source pull");
    await new Promise(r => setTimeout(r, 1000));
    console.log("source enqueue");
    c.enqueue("a");
    console.log("source pull done");
  },
  cancel(reason) {
    console.log("source cancel", reason);
  }
}, { highWaterMark: 0 });

var reader = rs.getReader();
reader.read().then((result) => console.log("read", result));
setTimeout(() => reader.cancel("hi"), 100);
```
This logs:
```
source pull
source cancel hi
read {value: undefined, done: true}
source enqueue
```
The source receives the `cancel()` call immediately, and the pending `read()` is fulfilled right away. When `pull()` eventually calls `enqueue()`, it'll throw an error (which the stream silently handles, since it has already become canceled):
```
TypeError: Failed to execute 'enqueue' on 'ReadableStreamDefaultController': Cannot enqueue a chunk into a closed readable stream
```

This also means my previous suggestion for passing an `AbortSignal` to `pull()` isn't really necessary. You can already do this yourself using `cancel()`:
```javascript
var rs = new ReadableStream({
  start() {
   this._controller = new AbortController();
  },
  async pull(c) {
    const response = await fetch("./more/data/please", {
      signal: this._controller.signal
    });
    const data = await response.json();
    c.enqueue(data);
  },
  cancel(reason) {
    this._controller.abort();
  }
}, { highWaterMark: 0 });
```

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