doc: add known issue for i40e statistics
doc: add known issue for i40e statistics

Rx statistic is incorrect when packet is oversize.

Signed-off-by: Jingjing Wu <64d28b617c3b4840f5ded1bfa611cba7fadfe5c7@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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