sched: Update yield() docs

/**

 * yield - yield the current processor to other threads.

 *

 * This is a shortcut for kernel-space yielding - it marks the

 * thread runnable and calls sys_sched_yield().

 * Do not ever use this function, there's a 99% chance you're doing it wrong.

 *

 * The scheduler is at all times free to pick the calling task as the most

 * eligible task to run, if removing the yield() call from your code breaks

 * it, its already broken.

 *

 * Typical broken usage is:

 *

 * while (!event)

 * 	yield();

 *

 * where one assumes that yield() will let 'the other' process run that will

 * make event true. If the current task is a SCHED_FIFO task that will never

 * happen. Never use yield() as a progress guarantee!!

 *

 * If you want to use yield() to wait for something, use wait_event().

 * If you want to use yield() to be 'nice' for others, use cond_resched().

 * If you still want to use yield(), do not!

 */

void __sched yield(void)

{