docs: powerpc: cxl.rst: mark two section titles as such

The User API chapter contains two sub-chapters. Mark them as
such.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Link: https://lore.kernel.org/r/190d67397cd63e419de8d85b92e8018d48e8c345.1586881715.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>