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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: donaldh