task: improve the example of poll_proceed

[ADD-SP]

Motivation

I noticed the example of poll_proceed is a bit long while thinking about the #7565 (comment).

I always prefer a short example because it helps people like me to grasp the basic usage in 10 seconds.

[Darksonn]

This is a good example, thanks.

[Darksonn]

Actually, I'm not so sure about the example. The branch returns Poll::Ready and we generally always .made_progress() when we return Ready. The other branch writes to the buffer but returns Pending, but if you wrote anything you should return Ready.

[ADD-SP]

Actually, I'm not so sure about the example. The branch returns Poll::Ready and we generally always .made_progress() when we return Ready. The other branch writes to the buffer but returns Pending, but if you wrote anything you should return Ready.

Ah, bad mistake, I have fixed it.

[ADD-SP]

I considered adding a reasonable Pending case, but I didn't find a reasonable one, so I chose to return Poll::Ready unconditionally.

[Darksonn]

What about having the example be "add coop to the futures mpsc channel"?

[ADD-SP]

What about having the example be "add coop to the futures mpsc channel"?

Looks still straightforward. I chose to not include the UnboundedSender, as the Sink requires too many methods to be implemented, which makes the example too long.

[Darksonn]

One last nit. Feel free to merge after this.