runtime: clarify the edge case of Builder::global_queue_interval()

[martin-g]

Motivation

If the interval is set to 1 then the schedulers will always execute tasks from the global queue. Tasks from the local queue will be executed only if the global queue is empty.

multi_thread scheduler:

tokio/tokio/src/runtime/scheduler/multi_thread/worker.rs

Lines 806 to 813 in 7127e25

	if self.tick % self.global_queue_interval == 0 {
	// Update the global queue interval, if needed
	self.tune_global_queue_interval(worker);
	worker
	.handle
	.next_remote_task()
	.or_else(|| self.next_local_task())

I think this clarification is needed because of

Setting the interval to a smaller value increases the fairness of the scheduler,
    /// at the cost of more synchronization overhead.

at https://github.com/tokio-rs/tokio/blob/7127e257a7d7025a5f6a62eb8aab81c2191f5cdb/tokio/src/runtime/builder.rs#L980C16-L981C54
With 1 there is no fairness anymore

Solution

Add a sentence to the documentation about the special case of global_queue_interval=1

[ADD-SP]

I think there is a space to improve the docs of global_queue_interval, but it is not required to do it in this PR. Thanks!