netlink: specs: add the multicast group name to spec

Add the multicast group's name to the YAML spec.
Without it YNL doesn't know how to subscribe to notifications.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250623231720.3124717-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/ethtool.yaml

@@ -2492,3 +2492,9 @@ operations:
         attributes:
           - header
           - events
+
+mcast-groups:
+  list:
+    -
+      name: monitor
+      c-define-name: ethtool-mcgrp-monitor-name