pktgen: doc were missing several config options

The pktgen.txt documentation over available config options were not complete.
Making the list complete by adding the following.

Pgcontrol commands:
 reset

Device commands:
 burst
 queue_map_min
 queue_map_max
 skb_priority
 tos
 traffic_class
 node
 spi
 dst6_max
 dst6_min
 vlan_cfi
 vlan_id
 vlan_p
 svlan_cfi
 svlan_id
 svlan_p

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>