[DOCS] Document archived settings (#78351)

Documents `archived.*` persistent cluster settings and index settings.
These settings are commonly produced during a major version upgrade.

Closes #28027

docs/reference/upgrade.asciidoc

@@ -106,3 +106,5 @@ include::upgrade/rolling_upgrade.asciidoc[]
 include::upgrade/cluster_restart.asciidoc[]
 
 include::upgrade/reindex_upgrade.asciidoc[]
+
+include::upgrade/archived-settings.asciidoc[]

docs/reference/upgrade/archived-settings.asciidoc

@@ -0,0 +1,76 @@
+[[archived-settings]]
+== Archived settings
+
+{es} typically removes support for deprecated settings at major version
+releases. If you upgrade a cluster with a deprecated persistent cluster setting
+to a version that no longer supports the setting, {es} automatically archives
+that setting. Similarly, if you upgrade a cluster that contains an index with an
+unsupported index setting, {es} archives the index setting.
+
+Archived settings start with the `archived.` prefix and are ignored by {es}.
+
+[discrete]
+[[archived-cluster-settings]]
+=== Archived cluster settings
+
+After an upgrade, you can view archived cluster settings using the
+<<cluster-get-settings,get cluster settings API>>.
+
+[source,console]
+----
+GET _cluster/settings?flat_settings=true&filter_path=persistent.archived*
+----
+
+You can remove archived cluster settings using the
+<<cluster-update-settings,update cluster settings API>>.
+
+[source,console]
+----
+PUT _cluster/settings
+{
+  "persistent": {
+    "archived.*": null
+  }
+}
+----
+
+{es} doesn't archive transient cluster settings or settings in
+`elasticsearch.yml`. If a node includes an unsupported setting in
+`elasticsearch.yml`, it will return an error at startup.
+
+[discrete]
+[[archived-index-settings]]
+=== Archived index settings
+
+IMPORTANT: Before you upgrade, remove any unsupported index settings from index
+and component templates. {es} doesn't archive unsupported index settings in
+templates during an upgrade. Attempts to use a template that contains an
+unsupported index setting will fail and return an error. This includes automated
+operations, such the {ilm-init} rollover action.
+
+You can view archived settings for an index using the <<indices-get-settings,get
+index settings API>>.
+
+[source,console]
+----
+GET my-index/_settings?flat_settings=true&filter_path=**.settings.archived*
+----
+// TEST[s/^/PUT my-index\n/]
+
+Removing archived index settings requires a reindex after the upgrade. However,
+reindexing can be resource intensive. Because {es} ignores archived settings,
+you can safely leave them in place if wanted.
+
+[source,console]
+----
+POST _reindex
+{
+  "source": {
+    "index": "my-index"
+  },
+  "dest": {
+    "index": "reindexed-v8-my-index"
+  }
+}
+----
+// TEST[s/^/PUT my-index\n/]