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>

Documentation/networking/devlink/index.rst

@@ -18,6 +18,10 @@ netlink commands.
 
 Drivers are encouraged to use the devlink instance lock for their own needs.
 
+Drivers need to be cautious when taking devlink instance lock and
+taking RTNL lock at the same time. Devlink instance lock needs to be taken
+first, only after that RTNL lock could be taken.
+
 Nested instances
 ----------------