Clarify explainer and address some less-complex issues

[jackbsteinberg]

Address Issues:

• Maybe extend Notification  #28
• Clarify behavior on background pages #27
• std-toast etc. look like custom element names #24
• Copy or reference some std-switch FAQs #22

[domenic]

I don't think this sufficiently addresses #27. It briefly mentions related things when talking about the Notifications API, but given the pattern folks have seen on Android, I think we should have an explicit section (maybe under additional resources / FAQs? I dunno) explaining that proposed toasts are an in-page element and not a system-level thing like notifications or like the old Android toasts API.

[domenic]

LGTM with nits!