rcu: Upgrade header comment for poll_state_synchronize_rcu()

This commit emphasizes the possibility of concurrent calls to
synchronize_rcu() and synchronize_rcu_expedited() causing one or
the other of the two grace periods being lost from the viewpoint of
poll_state_synchronize_rcu().

If you cannot afford to lose grace periods this way, you should
instead use the _full() variants of the polled RCU API, for
example, poll_state_synchronize_rcu_full().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>