Documentation: devlink: add a note about RTNL lock into locking section

Add a note describing the locking order of taking RTNL lock with devlink
instance lock.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>