Update OpenStack Client syntax for graphs (doc)
Update OpenStack Client syntax for graphs (doc)

The OSC syntax for specifying branching points in Service Graphs
is going to be different than initially designed. Instead of:

--port-chains pc1:pc2,pc3;pc2:pc4,pc5

Now it will look like:

--branching-point pc1:pc2,pc3 --branching-point pc2:pc4,pc5

Change-Id: Idcfdc0eacf373ed3961079edfdca948495c2ea1e
Partial-Bug: #1587486

openstack/networking-sfc,openstack/networking-sfc