[DOCS] Clarifying env variable substitution (#57370) (#57738)

* Clarifying environment variable substitution in the ES configuration YAML
* Update code snippet
* Remove extraneous quotes from string example
* Incorporating review feedback

Author: Adam Locke

docs/reference/setup/configuration.asciidoc

@@ -71,7 +71,7 @@ path.logs: /var/log/elasticsearch
 
 Environment variables referenced with the `${...}` notation within the
 configuration file will be replaced with the value of the environment
-variable, for instance:
+variable. For example:
 
 [source,yaml]
 --------------------------------------------------
@@ -109,3 +109,36 @@ Enter value for [node.name]:
 NOTE: Elasticsearch will not start if `${prompt.text}` or `${prompt.secret}`
 is used in the settings and the process is run as a service or in the background.
 
+Values for environment variables must be simple strings. Use a comma-separated string to provide values that Elasticsearch will parse as a list. For example, Elasticsearch will split the following string into a list of values for the `${HOSTNAME}` environment variable:
+
+[source,yaml]
+----
+export HOSTNAME=“host1,host2"
+----
+
+[discrete]
+[[cluster-setting-types]]
+=== Cluster and node setting types
+
+Cluster and node settings can be categorized based on how they are configured:
+
+[[dynamic-cluster-setting]]
+Dynamic::
+You can configure and update dynamic settings on a running cluster using the
+<<cluster-update-settings,cluster update settings API>>.
++
+You can also configure dynamic settings locally on an unstarted or shut down
+node using `elasticsearch.yml`.
++
+TIP: It’s best to set dynamic, cluster-wide settings with the cluster update
+settings API and use `elasticsearch.yml` only for local configurations. Using
+the cluster update settings API ensures the setting is the same on all nodes. If
+you accidentally configure different settings in `elasticsearch.yml` on
+different nodes, it can be difficult to notice discrepancies.
+
+[[static-cluster-setting]]
+Static::
+Static settings can only be configured on an unstarted or shut down node using
+`elasticsearch.yml`.
++
+Static settings must be set on every relevant node in the cluster.