Sending can yield the thread

[NathanRoyer]

When we call try_send on a sender, here's what I think is happening:

• ConcurrentQueue::push is called here
• If the queue is unbounded, Unbounded::push is called here
• If the queue reached its current capacity, busy_wait is called here
• Which either spins or yields the thread, as defined here

I find it weird that we end up yielding the thread in async code. If this is expected, I think it should be documented.