srcu: Point call_srcu() to call_rcu() for detailed memory ordering

This commit causes the call_srcu() kernel-doc header to reference that
of call_rcu() for detailed memory-ordering guarantees.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>