Cover std::task::yield in Rust Tasks and Communication Tutorial

[Sodel-the-Vociferous]

Since tasks are cooperatively scheduled, yielding is pretty important when using tasks with long running threads. It'd be good if the task and communication tutorial covered std::task::yield a bit, or at least mentioned it, to warn the gung-ho.

[pnkfelix]

part of #11755

[sanxiyn]

Triage. In #8560, yield was reserved as a keyword and std::task::yield was renamed to std::task::deschedule.

[steveklabnik]

Is this still relevant? Tasks have changed a lot recently, I am unsure what the current state is.

[alexcrichton]

In general the entire tasks tutorial needs a revamp most likely, it hasn't been scrutinized in a long time (I think).

[steveklabnik]

Seems good, I will consider this issue to be 'revamp the task tutorial' then.

[thestinger]

#17325 means this will become a very niche function. It will almost never be the right tool to reach for.