DownloadExtraInfo doesn't download bridge extra infos
There's some confusion about what these options do for bridge extra infos:
DownloadExtraInfo 1 FetchUselessDescriptors 1
We should clarify that they don't affect Tor's usual behaviour, which is to (eventually) download bridge descriptors, and never download bridge extra-infos.
Or we should make them download bridge extra-infos, and document that.