futex: Clarify comment for requeue_pi_wake_futex()

It's slightly confusing.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210902094414.618613025@linutronix.de