doc: add pdump limitation note
doc: add pdump limitation note

The note i.e. "The dpdk-pdump tool can only be used in
conjunction with a primary process which has the packet
capture framework initialized already" is added to
doc/guides/sample_app_ug/pdump.rst to facilitate
easy understanding on the usage of the tool.

Suggested-by: Jianfeng Tan <4339d9ba3a87a0d192e405945005ff86667f7b0c@intel.com>
Signed-off-by: Reshma Pattan <4d9eb35579760ed1c2db754a6b402039c9c8a1cb@intel.com>

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