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