pktgen: document ability to add same device to several threads

The pktgen.txt documentation still claimed that adding same device to
multiple threads were not supported, but it have been since 2008 via
commit e6fce5b916cd7 ("pktgen: multiqueue etc.").

Document this and describe the naming scheme dev@X, as the procfile name
still need to be unique.

Fixes: e6fce5b916cd7 ("pktgen: multiqueue etc.")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>