docs: add note about cluster renaming

Signed-off-by: Justin Garrison <justin.garrison@siderolabs.com>

Author: rothgar

public/omni/getting-started/create-a-cluster.mdx

@@ -15,6 +15,8 @@ This guide shows you how to create a cluster from registered machines.
 
     <Info>
       Note that disk encryption can only be enabled during cluster creation. Enabling this checkbox will configure the cluster to use Omni as a [Key Management Server](../security-and-authentication/omni-kms-disk-encryption), and local disk access will not the possible unless the machine is connected to Omni.
+
+      It is not possible to rename a cluster after it has been created.
     </Info>
 
     From the Available Machines list, select the role (CP for control plane, WX (where X is a number) for workers) for each machine you would like to be a member of the cluster. Optionally, choose the install disk for the OS from the dropdown menu for each machine.