[OSDOCS-3566]: Add admonition about lack of machine set support for control planes

Author: jeana-redhat

modules/machine-api-overview.adoc

@@ -23,6 +23,11 @@ The two primary resources are:
 Machines:: A fundamental unit that describes the host for a node. A machine has a `providerSpec` specification, which describes the types of compute nodes that are offered for different cloud platforms. For example, a machine type for a worker node on Amazon Web Services (AWS) might define a specific machine type and required metadata.
 
 Machine sets:: `MachineSet` resources are groups of machines. Machine sets are to machines as replica sets are to pods. If you need more machines or must scale them down, you change the *replicas* field on the machine set to meet your compute need.
++
+[WARNING]
+====
+Control plane machines cannot be managed by machine sets.
+====
 
 The following custom resources add more capabilities to your cluster: