add docs note on popup notifications

[jantman]

I don't do a whole lot of frontend development, and haven't really followed the state of browser APIs in quite a while.

Originally, when I enabled Google Hangouts and Google Voice, I thought that the lack of native (system tray) popup notifications from them was Franz's fault. It was only after a lot of research and experimentation (and running some service clients directly in Chrome) that I realized the services that provide popup notifications for new messages are doing that natively, using the Web Notifications API.

I thought it would be nice to add some information about this to the plugin docs, for the benefit of anyone else who's as confused as I was by that.