Add bit about semantics of push notification delivery guarantees

I've added the delivery guarantees notes as requested. I hope it makes sense. For further details you want to read Section 5 for the Thialfi paper [1], which goes into details.

It should be normative because its a guarantee, but I've put it in the non-normative description section where it makes sense. It is sort of hard to understand without understanding the semantics of the underlying protocol, so it may make sense to move this to the spec that defines the protocol between user agent and push server.

Not sure if I should add myself to authors.

[1] http://static.googleusercontent.com/external_content/untrusted_dlcp/research.google.com/en//pubs/archive/37474.pdf