rcu: Update list of rcu_future_grace_period() trace events

Reworking grace-period initiation and funnel locking added new
rcu_future_grace_period() trace events, so this commit updates the
rcu_future_grace_period() trace event's header comment accordingly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>