[ipam][docs] clarify multihost setup

[rade]

The IPAM docs are rather confusing/unclear on how to get a multihost setup running...

1. The only example of launching IPAM is weave launch -iprange 10.2.3.0/24. So there is a good chance drive-by evaluators will run that on just one host, despite what it says in the text further down.
2. Ditto, but if they've paid closer attention they will just add the -iprange ... to every weave launch command. Will that work on the typical drive-by cases, e.g. launch 1st node with no addresses, 2nd node pointing at 1st, 3rd node pointing at 2nd (or, alternatively, pointing at 2nd and 3rd), etc? Assuming a human doing this, i.e. plenty of time passing between the steps.
3. "you must either supply the list of all peers in the network to weave launch or add the -initpeercount flag to specify how many peers there will be" - this either conflicts with, or at the very least causes puzzlement when juxtaposed with the only example given, which does not specify any peers nor -initpeercount.

I think as a minimum we should show an example of a 3 host IPAM setup for "incremental drive-by-evaluation" (assuming (2) above actually works), and another example for automated deployment.