timekeeping: Add missing parameter docs for pvclock_gtod_[un]register_notifier()

The kernel-doc parser complains about:
 kernel/time/timekeeping.c:651: warning: Function parameter or member
 'nb' not described in 'pvclock_gtod_register_notifier'
 kernel/time/timekeeping.c:670: warning: Function parameter or member
 'nb' not described in 'pvclock_gtod_unregister_notifier'

Add the missing parameter explanations.

[ tglx: Massaged changelog ]
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1605252275-63652-3-git-send-email-alex.shi@linux.alibaba.com

kernel/time/timekeeping.c

@@ -663,6 +663,7 @@ static void update_pvclock_gtod(struct timekeeper *tk, bool was_set)
 
 /**
  * pvclock_gtod_register_notifier - register a pvclock timedata update listener
+ * @nb: Pointer to the notifier block to register
  */
 int pvclock_gtod_register_notifier(struct notifier_block *nb)
 {
@@ -682,6 +683,7 @@ EXPORT_SYMBOL_GPL(pvclock_gtod_register_notifier);
 /**
  * pvclock_gtod_unregister_notifier - unregister a pvclock
  * timedata update listener
+ * @nb: Pointer to the notifier block to unregister
  */
 int pvclock_gtod_unregister_notifier(struct notifier_block *nb)
 {