packet: doc: describe PACKET_MMAP with one packet socket for rx and tx

Document how to use one AF_PACKET mmap socket for RX and TX.

Signed-off-by: Norbert van Bolhuis <nvbolhuis@aimvalley.nl>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>