Document join is concurrent but not parallel

[elbaro]

It should be documeneted in join, join3, join4, ..join5 that they are concurrently run not run in parallel,
and how to run them in parallel.

https://docs.rs/tokio/latest/tokio/macro.join.html

[akauppi]

This entry (from 2021) refers to Tokio, but this repo is futures-rs. Perhaps these were closer at that point, but currently I think this could be closed?

[elbaro]

This issue is about futures-rs. The link is an example of how Tokio documented tokio::join!.