tick/nohz: Improve tick_nohz_get_next_hrtimer() kerneldoc

Make the tick_nohz_get_next_hrtimer() kerneldoc comment state clearly
that the function may return negative numbers.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/time/tick-sched.c

@@ -1124,7 +1124,11 @@ ktime_t tick_nohz_get_next_hrtimer(void)
  * tick_nohz_get_sleep_length - return the expected length of the current sleep
  * @delta_next: duration until the next event if the tick cannot be stopped
  *
- * Called from power state control code with interrupts disabled
+ * Called from power state control code with interrupts disabled.
+ *
+ * The return value of this function and/or the value returned by it through the
+ * @delta_next pointer can be negative which must be taken into account by its
+ * callers.
  */
 ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
 {