doc/netlink/specs: Add sub-message type to rt_link family

Start using sub-message selectors in the rt_link spec for the
link-specific 'data' and 'slave-data' attributes.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-7-donald.hunter@gmail.com


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