doc: update metadata feature in mlx5 guide
doc: update metadata feature in mlx5 guide

Legacy Verbs supports MARK and FLAG metadata actions only.

Signed-off-by: Viacheslav Ovsiienko <5b8db9bb93a9a9aefc8828f79604d1fe963a53dd@mellanox.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk