pvecm: re-add "multiple cluster in same net" section

While yes, there's a note covering this, users have had often
questions about it, or if it's possible at all, so it deserves its
own section, it's more visible and can be better found this way.

Adapt for corosync 3

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

pvecm.adoc

@@ -127,6 +127,19 @@ To check the state of your cluster use:
  hp1# pvecm status
 ----
 
+Multiple Clusters In Same Network
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to create multiple clusters in the same physical or logical
+network. Each such cluster must have a unique name, this does not only helps
+admins to distinguish on which cluster they currently operate, it is also
+required to avoid possible clashes in the cluster communication stack.
+
+While the bandwidth requirement of a corosync cluster is relatively low, the
+latency of packages and the package per second (PPS) rate is the limiting
+factor. Different clusters in the same network can compete with each other for
+these resources, so it may still make sense to use separate physical network
+infrastructure for bigger clusters.
 
 [[pvecm_join_node_to_cluster]]
 Adding Nodes to the Cluster