Documentation: net/mlx5: Use bullet and definition lists for vnic counters description

"vnic reporter" section contains unformatted description for vnic
counters, which is rendered as one long paragraph instead of list.

Use bullet and definition lists to match other lists.

Fixes: b0bc615d

 ("net/mlx5: Add vnic devlink health reporter to PFs/VFs")
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>