simulation docs improvement

[jeffallen]

The docs for "type Simulation" at https://godoc.org/github.com/dedis/onet#Simulation do not clearly explain that Setup() happens on the simulation driving computer, Node() runs on each node, and Run() runs on a node, but only on the root node.

Other documentation improvements related to simulation are also likely possible, the person who does this task should do a deep dive and improve as much as is needed.