devlink: Add comments to use netlink gen tool

Add the comment to remind people not to manually modify
the net/devlink/netlink_gen.c, but to use tools/net/ynl/ynl-regen.sh
to generate it.
Signed-off-by: William Tu <witu@nvidia.com>
Suggested-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240310145503.32721-1-witu@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/devlink.h

@@ -614,7 +614,10 @@ enum devlink_attr {
 
 	DEVLINK_ATTR_REGION_DIRECT,		/* flag */
 
-	/* add new attributes above here, update the policy in devlink.c */
+	/* Add new attributes above here, update the spec in
+	 * Documentation/netlink/specs/devlink.yaml and re-generate
+	 * net/devlink/netlink_gen.c.
+	 */
 
 	__DEVLINK_ATTR_MAX,
 	DEVLINK_ATTR_MAX = __DEVLINK_ATTR_MAX - 1