doc: add limitation for ConnectX-4 with L2 in mlx5 guide

doc: add limitation for ConnectX-4 with L2 in mlx5 guide

ConnectX-4 and ConnectX-4 Lx NICs require all L2 headers of transmitted
packets to be inlined. By default only first 18 bytes are inlined,
which is insufficient if additional encapsulation is used, like Q-in-Q.
Thus, default settings caused such traffic to be dropepd on Tx.
Document a recommendation to increase inlined data size in such cases.

Fixes: 505f1fe426d3 ("net/mlx5: add Tx devargs")
Cc: stable@dpdk.org
Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>