Re: [ServiceWorker] CacheStorage.delete() does not specify how to handle currently referenced Cache (#538)

We should add a note on the lifetime of cached items on disk (or wherever they're stored).

A cached item should remain on disk until:

* its cache has been dereferenced from `NameToCacheMap`
* no references exist to the cache
* no references exist to the request/response

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https://github.com/slightlyoff/ServiceWorker/issues/538#issuecomment-123503353