doc: add note for --txqflags in testpmd
doc: add note for --txqflags in testpmd

Since txqflags is now set from the default rx/tx configuration,
depending on the PMD, it might not be 0.
Therefore, user has to overwrite it with --txqflags 0.

Signed-off-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>
Acked-by: Siobhan Butler <8cf2da75f16c15b95e66f52a35c85d9ba5dc91d6@intel.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk