[slightlyoff/ServiceWorker] clarify that Clients.matchAll() only returns Window Client objects with active documents (#888)

The `Clients.matchAll()` spec depends on their being an active document in the browsing context in a couple steps, but isn't really clear what to do if there is no active document.  I think we should be more explicit these windows are excluded from the list.

Related: https://github.com/whatwg/html/issues/1073

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