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

Since changeset 58ad30cf ("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: 58ad30cf ("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>