[DOCS] Update X-Pack installation re TLS changes (elastic/x-pack-elasticsearch#2523)
* [DOCS] Update xpack installation re TLS changes
* [DOCS] Clarify multi-node TLS requirement
Original commit: elastic/x-pack-elasticsearch@f2dd0750bc1cf4808ad351781326de534a2a3760
@@ -139,11 +139,17 @@ the correct value for your environment, you may consider setting the value to
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. Configure passwords and SSL/TLS.
-.. If you have more than one node or a single node that listens on an external
-interface, you must configure SSL/TLS for inter-node communication. Single-node
-instances that use a loopback interface do not have this requirement. For more
+.. If you have a non-trial license and you want to use {security}, you must
+configure SSL/TLS for internode-communication.
++
+--
+NOTE: This requirement applies to clusters with more than one node and to
+clusters with a single node that listens on an external interface. Single-node
+clusters that use a loopback interface do not have this requirement. For more
information, see
{xpack-ref}/encrypting-communications.html[Encrypting Communications].
+
+--
... Generate node certificates. For example, you can use the `certgen` command
line tool to generate a certificate authority and signed certificates for your
nodes.
