dpif-dpdk: Add some documentation

Change-Id: I732bde5e81b9decab439938eb2572c30fc83022e
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Kevin Traynor <kevin.traynor@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>