netlink: specs: mptcp: add missing 'server-side' attr

netlink: specs: mptcp: add missing 'server-side' attr

This attribute is added with the 'created' and 'established' events, but
the documentation didn't mention it.

The documentation in the UAPI header has been auto-generated by:

  ./tools/net/ynl/ynl-regen.sh

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241221-net-mptcp-netlink-specs-pm-doc-fixes-v2-1-e54f2db3f844@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>