rcu: Update synchronize_rcu_mult() comment for call_rcu_hurry()

Those who have worked with RCU for some time will naturally think in
terms of the long-standing call_rcu() API rather than the much newer
call_rcu_hurry() API.  But it is call_rcu_hurry() that you should normally
pass to synchronize_rcu_mult().  This commit therefore updates the header
comment to point this out.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>