docs: add cluster-to-cluster communications to external threat overview

website/content/docs/internals/security.mdx

@@ -23,7 +23,7 @@ The following are the various parts of the Vault threat model:
 
 - Eavesdropping on any Vault communication. Client communication with Vault
   should be secure from eavesdropping as well as communication from Vault to
-  its storage backend.
+  its storage backend or between Vault cluster nodes.
 
 - Tampering with data at rest or in transit. Any tampering should be detectable
   and cause Vault to abort processing of the transaction.
@@ -85,6 +85,13 @@ require that a client provides a client token for every request which is used
 to identify the client. A client that does not provide their token is only
 permitted to make login requests.
 
+All server-to-server traffic between Vault instances within a cluster (i.e,
+high availability, enterprise replication or integrated storage) uses
+mutually-authenticated TLS to ensure the confidentiality and integrity of data
+in transit. Nodes are authenticated prior to joining the cluster, by an
+[unseal challenge](/docs/concepts/integrated-storage#vault-networking-recap) or
+a [one-time-use activation token](/docs/enterprise/replication#security-model).
+
 The storage backends used by Vault are also untrusted by design. Vault uses a
 security barrier for all requests made to the backend. The security barrier
 automatically encrypts all data leaving Vault using a 256-bit [Advanced