Document that Eliot does no disk I/O in logging thread (except that Destinations do so actually this is a little silly)
This isn't really the case - at least not currently - is it? If you add a FileDestination to the global Logger's Destinations then whatever thread calls Logger.write will call Destinations.send will call FileDestination.__call__ will call file.write. As far as I can tell, if you want any special thread-related behavior at all you have to take extra steps to arrange for it.
Perhaps this ticket dates from a time when ThreadedWriter (or ThreadedFileWriter) was more central?
Yeah... it's more that outside of destinations there's no file I/O, but given what destinations do that's sort of silly.
Definitely want to have ThreadedWriter that isn't Twisted-specific. I wonder if new non-broken queue in Python 3.7 has been backported.