docs: add two todos

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
trafgen: when using a inter-pkt-gap > 0, use only 1 core

It's slow anyway, so no need to multithread; therefore, we have
a correct interpacket gap.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>