doc: add limitations for i40e PMD
doc: add limitations for i40e PMD

This patch adds "Limitations or Known issues" section for
i40e PMD, including two items:
1. MPLS packet classification on X710/XL710
2. 16 Byte Descriptor cannot be used on DPDK VF
3. Link down with i40e kernel driver after DPDK application exist

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