doc: add metering limitation in mlx5 guide

doc: add metering limitation in mlx5 guide

A meter policy with RSS/Queue action is not supported
when dv_xmeta_en enabled.

When dv_xmeta_en enabled in legacy creating flow,
it will split into two flows
(one set_tag with jump flow and one RSS/queue action flow).
For meter policy as termination table,
it cannot split flow and
cannot support when dv_xmeta_en enabled.

Fixes: 51ec04dc7bcf ("net/mlx5: connect meter policy to created flows")
Cc: stable@dpdk.org
Signed-off-by: Li Zhang <lizh@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>