Docs/RCU/rculist_nulls: Fix text about atomic_set_release()

The document says we can avoid extra _release() in insert function when
hlist_nulls is used, but that's not true[1].  Drop it.

[1] https://lore.kernel.org/rcu/46440869-644a-4982-b790-b71b43976c66@paulmck-laptop/



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