Add a section for sub-messages to the generated .rst files.
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-11-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>