doc: revise sample testpmd flow commands
doc: revise sample testpmd flow commands

Stop port before enabling QinQ.
Add commands to set inner and outer TPID's and start port.
Remove TPID's from flow validate and and flow create commands.

Signed-off-by: Bernard Iremonger <233eeff376ad0c06f66f06c5522ea3c4feb30d3b@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