[DOCS] Secure settings specified per node (#31621)

Make it clear that secure settings have to be set
on each cluster node.

docs/reference/setup/secure-settings.asciidoc

@@ -1,5 +1,5 @@
 [[secure-settings]]
-=== Secure Settings
+=== Secure settings
 
 Some settings are sensitive, and relying on filesystem permissions to protect
 their values is not sufficient. For this use case, Elasticsearch provides a
@@ -16,6 +16,10 @@ Elasticsearch.
 NOTE: The elasticsearch keystore currently only provides obfuscation. In the future,
 password protection will be added.
 
+These settings, just like the regular ones in the `elasticsearch.yml` config file,
+need to be specified on each node in the cluster. Currently, all secure settings
+are node-specific settings that must have the same value on every node.
+
 [float]
 [[creating-keystore]]
 === Creating the keystore