rculist_nulls: Add docbook comments

This patch adds docbook comment headers for hlist_nulls_first_rcu()
and hlist_nulls_next_rcu() in rculist_nulls.h.

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>