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>

include/linux/rculist_nulls.h

@@ -38,9 +38,17 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
 	}
 }
 
+/**
+ * hlist_nulls_first_rcu - returns the first element of the hash list.
+ * @head: the head of the list.
+ */
 #define hlist_nulls_first_rcu(head) \
 	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
 
+/**
+ * hlist_nulls_next_rcu - returns the element of the list after @node.
+ * @node: element of the list.
+ */
 #define hlist_nulls_next_rcu(node) \
 	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))