Clarify encryption key lifetime and intention

[beverloo]

Fixes #212

[beverloo]

FYI @PascalReintjes

I'm fiddling around with introducing a concept of "invalidating" push subscriptions in preparation of the bigger pushsubscriptionchange PR. That's definitely relevant since we'll want to detail what happens when the UA has to change the encryption keys, but I don't think that has to block the clarifications.

[martinthomson]

Thanks Peter, I agree that the invalidation text really does need to go with the event refactor work.

[PascalReintjens]

Thanks @beverloo, that makes it perfectly clear :)