[w3c/ServiceWorker] clarify that Client "creation order" means when reserved Client created (#1078)

The `Clients.matchAll()` method says to sort Clients in creation order (among other things).  I just want to confirm this means the sorted by the time the reserved client was created.  Alternatively, it could mean when the environment backing the Client actually became Execution Ready.

I favor the reserved client creation time because it provides a reasonable sort order for reserved clients included in the list.

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