Clarifiy unexpected Packet private CIDR behaviour (#163)

Attempts to mitigate issue #159 by stating that this CIDR might not be stable

docs/flatcar-linux/packet.md

@@ -110,6 +110,7 @@ module "controller" {
   # Check yours from the `IPs & Networks` tab from your Packet.net account.
   # If an IP block is not allocated yet, try provisioning an instance from the console in
   # that region. Packet will allocate a public IP CIDR.
+  # Note: Packet does not guarantee this CIDR to be stable if there are no servers deployed in the project and region
   node_private_cidr = "10.128.156.0/25"
 }