Add a description of the "Tubes" use of navigator.connect.

@slightlyoff @dglazkov @jakeleichtling, I'd like to include this use case so that folks don't feel like we're trying to sneak it through. Can you help me describe it correctly?
When I was talking to @sowbug about how to make sure that we can remove these endpoints, the idea of a rotating path was the only one that seemed plausible. Otherwise there's nothing really different between removing a URL-built-into-Chrome vs removing a Javascript-function-built-into-Chrome. Both have the same chance of breaking the web. I can certainly remove it, but I'm worried that'll undeservedly bias people against the idea when it comes up again.

Apologies for implying that this was part of the original Tubes proposal.