netlink: specs: add doc start markers to yaml

Clean up all document-start warnings reported by yamllint in the
netlink specs:

    warning  missing document start "---"  (document-start)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> # mptcp_pm.yaml
Link: https://patch.msgid.link/20250610125944.85265-2-donald.hunter@gmail.com


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