net/mlx5: Document support for RoCE HCA disablement capability

Some mlx5 devices are capable of disabling RoCE. In this situation,
disablement does not need to be handled at the driver level.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>