Docs/RCU/rculist_nulls: Specify type of the object in examples

The type of 'obj' in example code of rculist_nulls.rst is implicit.
Provide the specific type of it before the example code.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/rcu/43943609-f80c-4b6a-9844-994eef800757@paulmck-laptop/
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: sjp38

Documentation/RCU/rculist_nulls.rst

@@ -18,7 +18,16 @@ to solve following problem.
 
 Without 'nulls', a typical RCU linked list managing objects which are
 allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can use the following
-algorithms:
+algorithms.  Following examples assume 'obj' is a pointer to such
+objects, which is having below type.
+
+::
+
+  struct object {
+    struct hlist_node obj_node;
+    atomic_t refcnt;
+    unsigned int key;
+  };
 
 1) Lookup algorithm
 -------------------
@@ -144,6 +153,9 @@ the beginning. If the object was moved to the same chain,
 then the reader doesn't care: It might occasionally
 scan the list again without harm.
 
+Note that using hlist_nulls means the type of 'obj_node' field of
+'struct object' becomes 'struct hlist_nulls_node'.
+
 
 1) lookup algorithm
 -------------------