Add a small clarification on using sub-CA for a cluster (neo4j#1790) (neo4j#1794)

Author: NataliaIvakina

modules/ROOT/pages/clustering/setup/encryption.adoc

@@ -66,8 +66,8 @@ This means that the server now has the capability of establishing trust with oth
 [CAUTION]
 ====
 Be sure to exercise caution when using CA certificates in the `trusted` directory, as any certificates signed by that CA are then trusted to join the cluster.
-For this reason, never use a public CA to sign certificates for your cluster.
-Instead, use an intermediate certificate or a CA certificate which originates from and is controlled by your organization.
+Never use a public CA or your internal root CA to sign certificates for your cluster.
+Instead, use an intermediate certificate or a CA certificate which originates from and is controlled by your organization, and is only used for that specific cluster.
 ====
 
 In this example a mutual authentication setup is deployed, which means that both ends of a channel have to authenticate.