concurrency: `tokio::select` vs. `futures::select`

Currently when talking about combining futures, we introduce the `futures::join` macro and `futures::join_all` function, then `tokio::select`. But there is also a [`select`](https://docs.rs/futures/latest/futures/macro.select.html) macro in the futures library itself, though it has slightly different semantics and requirements: https://stackoverflow.com/questions/60811657/what-is-the-difference-between-futuresselect-and-tokioselect

In particular, the `futures` version requires futures to implement the `FusedFuture` trait and accesses them by mutable reference while the `tokio` variant moves futures (so users may instead pass `&mut f`) and does not require fusing.

We should at least mention the existence of the non-Tokio version so things don't seem so magical.