doc/netlink: Document the sub-message format for netlink-raw

Document the spec format used by netlink-raw families like rt and tc.

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-4-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>