doc: add missing loopback option in testpmd guide

doc: add missing loopback option in testpmd guide

This patch adds a missing topology "loop" mode for port-topology
option in testpmd guide. The testpmd "loop" option
(PORT_TOPOLOGY_LOOP)  was added to testpmd parameter
port-topology back in 2014, but it seems that it was never added
to the testpmd guide. This patch adds it in the testpmd guide.

Fixes: 3e2006d6186c ("app/testpmd: add loopback topology")
Cc: stable@dpdk.org
Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>