rcu: Add CONFIG_RCU_LAZY delays to call_rcu() kernel-doc header

This commit adds a description of the energy-efficiency delays that
call_rcu() can impose, along with a pointer to call_rcu_hurry() for
latency-sensitive kernel code.

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