doc: update timestamp validity for latency measurement
doc: update timestamp validity for latency measurement

Updated the doc on how packets are marked to identify
their timestamp as valid and considered for latency
measurement.

Suggested-by: Bao-Long Tran <dd4b43c1703aa83532bb2c360f81427f3c5605a9@viettel.com.vn>
Signed-off-by: Reshma Pattan <4d9eb35579760ed1c2db754a6b402039c9c8a1cb@intel.com>

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