doc: Clarify use of slab constructors and SLAB_TYPESAFE_BY_RCU

This commit explicitly states that you should initialize any locks to
be used by readers in your SLAB_TYPESAFE_BY_RCU constructor.

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