[push-api] pushregistrationlost isn't documented (#149)

We keep talking about this, and I think we still need it.  The `pushsubscriptionchange` event is fine for the case where the browser is able to seamlessly switch out the current subscription for another.  However, there are plenty of cases where the subscription

We could use `pushsubscriptionchange` and then return *null* out of `getSubscription()`, but that seems janky.

This relates to #116 and #61.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/149