doc: add metadata limitation in mlx5 guide
doc: add metadata limitation in mlx5 guide

The metadata limitation is described.

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

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