doc: add LRO size limitation in mlx5 guide

Maximal LRO message size must be multiply of 256.
Otherwise, TCP payload may not fit into a single WQE.

Fixes: 1c7e57f9bd ("net/mlx5: set maximum LRO packet size")
Cc: stable@dpdk.org

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>

doc/guides/nics/mlx5.rst

@@ -404,6 +404,8 @@ Limitations
     TCP header (122B).
   - Rx queue with LRO offload enabled, receiving a non-LRO packet, can forward
     it with size limited to max LRO size, not to max RX packet length.
+  - The driver rounds down the port configuration value ``max_lro_pkt_size``
+    (from ``rte_eth_rxmode``) to a multiple of 256 due to hardware limitation.
   - LRO can be used with outer header of TCP packets of the standard format:
         eth (with or without vlan) / ipv4 or ipv6 / tcp / payload