Design document on `ThreadedRunner` outdated

[mih]

https://docs.datalad.org/en/stable/design/threaded_runner.html

I did not go through the entire text, but it claims that we use asyncio.protocols.SubprocessProtocol, which is no longer the case (since datalad v0.17, 2022-07-07).

Ping @christian-monch

[yarikoptic-gitmate]

Issue fixed in 0.19.4