[w3c/ServiceWorker] clarify that manually constructed ExtendableEvent objects may not extend lifetime via waitUntil() (#1040)

Currently its possible to do this in the spec:

```
let e = new ExtendableEvent('foo');
e.waitUntil(new Promise());
```

By default the extensions allowed flag is set, so this does not throw an InvalidStateError.

We should be clear, though, that this does not actually extend the lifetime of the SW.  Currently the spec is written in a way that suggests that this might imbue some keep-alive budget to the SW.

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