future::select example

[keepsimple1]

I felt the current example in future::select documentation is too advanced / complicated for a new user. I think that whoever trying to read this example is likely a new user.

futures-rs/futures-util/src/future/select.rs

Lines 31 to 45 in 95d5801

	/// use futures::future::{self, Either, Future, FutureExt};
	///
	/// // A poor-man's join implemented on top of select
	///
	/// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
	/// where A: Future + Unpin,
	/// B: Future + Unpin,
	/// {
	/// future::select(a, b).then(|either| {
	/// match either {
	/// Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
	/// Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
	/// }
	/// })
	/// }

Is it possible to have a simpler / more basic example for this function? Particularly, it will be great if the example can clearly show what the value(s) returned in each arm are.

[ghost]

I'd also love an example that is:

1. more explicit about what the elements of the pairs within the Either are, you have to do a lot of parsing of this example to tell what x, a, y, and b are meant to mean

2. uses this function in a more realistic scenario than reimplementing something select doesn't do

[taiki-e]

Done in #2182