Make it clearer that the Tor Browser update download is happening over Tor

The new update download dialog in Tor Browser 4.0 looks like an operating system dialog that is making a network connection. Normally when one sees an operating system dialog making a network connection, whatever update or whatever is happening doesn't happen over Tor. With Tor Browser, that's not true, as the download does indeed take place over Tor. We should make this clear to our users to assuage doubt.