Book: explain fuse and select

The `select!` macro together with `FuturesExt::fuse`, `FusedStream` and `FusedFuture` is a pretty powerful mechanism which isn't all that easy to understand. The [current version of the doc](https://github.com/async-rs/async-std/blob/0156dc879bc67e313ef123978a8edfb24ae36fae/docs/src/tutorial/handling_disconnection.md) only mentions `select!` in passing and doesn't address `fuse` at all.

Linking to the [respective section in the async book](https://rust-lang.github.io/async-book/06_multiple_futures/03_select.html), or to the trait and function definitions might help, although I am not sure how stable they are.