Add kernel doc annotation for HRTIMER_MODE_HARD.

Fixes: ae6683d8 ("hrtimer: Introduce HARD expiry mode")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190823113845.12125-2-bigeasy@linutronix.de