devlink: document devlink_rel_nested_in_notify() function

Add a documentation for devlink_rel_nested_in_notify() describing the
devlink instance locking consequences.

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