[w3c/push-api] Clarify purpose expirationTime (#340)

The purpose of PublicationSubscription's `expirationTime` attribute is a bit unclear in the spec. [Elsewhere](https://phabricator.services.mozilla.com/D128046), @martinthomson wrote:

> the spec needs is a description of what it means to set this value, specifically that the *browser* decides what the value is and that it indicates to sites whether the subscription will automatically end and - if it ends - when. 



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