[w3c/ServiceWorker] clarify when FetchEvent.clientId will or will not be set for navigations (#1267)

Over in #1266 the question of when exactly `FetchEvent.clientId` will or will not be populated for navigations came up.

It seems it must not be populated for:

* If the navigation is initiated from a cross-origin Client
* If there is no Client at all, like the user entering a URL in the tab and pressing enter.

We could potentially should a clientId for:

* Same-origin client setting window.location
* Same-origin client setting iframe.src
* Same-origin client calling window.open()
* Same-origin client calling clients.openWindow()

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