doc: comment testpmd checksum option
doc: comment testpmd checksum option

Add a comment for outer-ip option in csum command.
Set outer-ip option only when the packet is a IPv4 packet.

Signed-off-by: Jijiang Liu <10b95deab7df654d0c21c8c98b54c9a1ecf2a5c0@intel.com>
Acked-by: Marvin Liu <d89ea4150d91f674af88bbc382d184dd42ea906c@intel.com>

mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,venkynv/dpdk-mirror,tsphillips/dpdk-fork,mixja/dpdk,tsphillips/dpdk-fork,msune/dpdk,venkynv/dpdk-mirror,msune/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,mixja/dpdk