docs: RCU: Don't duplicate chapter names in rculist_nulls.rst

Since changeset 58ad30cf91 ("docs: fix reference to core-api/namespaces.rst"),
auto-references for chapters are generated. This is a nice feature, but
has a drawback: no chapters can have the same sumber.

So, we need to add two higher hierarchy chapters on this document,
in order to avoid such duplication.

Fixes: 58ad30cf91 ("docs: fix reference to core-api/namespaces.rst")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/rculist_nulls.rst

@@ -10,6 +10,9 @@ objects using SLAB_TYPESAFE_BY_RCU allocations.
 
 Please read the basics in Documentation/RCU/listRCU.rst
 
+Using 'nulls'
+=============
+
 Using special makers (called 'nulls') is a convenient way
 to solve following problem :
 
@@ -126,6 +129,9 @@ very very fast (before the end of RCU grace period)
 
 --------------------------------------------------------------------------
 
+Avoiding extra smp_rmb()
+========================
+
 With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
 and extra smp_wmb() in insert function.