Update docs for service worker integration

[gaearon]

See #3817.
Need to update them to document the new behavior.

[jeffposnick]

@Timer @gaearon

I'm happy to help, whether it's reviewing the updates written by someone else, or alternatively I'm happy to take a stab at the revised docs myself.

[gaearon]

I'd appreciate if you could take this and send a PR. Thank you very much!