[w3c/manifest] Clarify service worker member, and when it installs (#640)

@GrosSacASac wrote: 

If I understand correctly, the service worker declaration inside the manifest has no consequence unless the web app is installed without directly visting the web app url.(with an web app store ?)

I ask because I have an app where the service worker is only installed after the users checks the checkbox USE OFFLINE. I don't want the manifest to install the service worker on normal page load without user consent first.

I came here because https://www.w3.org/TR/appmanifest/#serviceworker-member words are confusing.

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https://github.com/w3c/manifest/issues/640