[docs] Add 9.0 release notes placeholders

docs/reference/cluster/transient-settings-migration-guide.asciidoc

@@ -0,0 +1,116 @@
+[[transient-settings-migration-guide]]
+==== Transient settings migration guide
+
+////
+[source,console]
+----
+PUT _cluster/settings
+{
+  "transient": {
+    "cluster.indices.close.enable": false,
+    "indices.recovery.max_bytes_per_sec": "50mb"
+  }
+}
+----
+////
+
+We no longer recommend using transient cluster settings. You can use transient
+settings to make temporary configuration changes to a cluster. However, a
+cluster restart or cluster instability can unexpectedly clear these settings,
+leading to a potentially undesired cluster configuration.
+
+To avoid this risk, reset any transient settings you've configured on
+your cluster. Convert any transient setting you'd like to keep to a persistent
+setting, which persists across cluster restarts and cluster instability. You
+should also update any custom workflows and applications to use persistent
+settings instead of transient settings.
+
+IMPORTANT: Some Elastic products may use transient settings when performing
+specific operations. Only reset transient settings configured by you, your
+users, or your custom workflows and applications.
+
+To reset and convert transient settings:
+
+. Get a list of any configured transient settings using the
+<<cluster-get-settings,cluster get settings API>>.
++
+[source,console]
+----
+GET _cluster/settings?flat_settings=true&filter_path=transient
+----
+// TEST[continued]
++
+The API returns transient settings in the `transient` object. If this object is
+empty, your cluster has no transient settings, and you can skip the remaining
+steps.
++
+[source,console-result]
+----
+{
+  "persistent": { ... },
+  "transient": {
+    "cluster.indices.close.enable": "false",
+    "indices.recovery.max_bytes_per_sec": "50mb"
+  }
+}
+----
+// TESTRESPONSE[s/"persistent": \{ \.\.\. \},//]
+
+. Copy any settings you'd like to convert into the `persistent` object of a
+<<cluster-update-settings,cluster update settings API>> request. In the same
+request, reset any transient settings by assigning them a `null` value.
++
+[source,console]
+----
+PUT _cluster/settings
+{
+  "persistent": {
+    "cluster.indices.close.enable": false,
+    "indices.recovery.max_bytes_per_sec": "50mb"
+  },
+  "transient": {
+    "*": null
+  }
+}
+----
+// TEST[continued]
+
+. Use the <<cluster-get-settings,cluster get settings API>> to confirm your
+cluster has no remaining transient settings.
++
+[source,console]
+----
+GET _cluster/settings?flat_settings=true
+----
+// TEST[continued]
++
+If the `transient` object is empty, your cluster has no transient settings.
++
+[source,console-result]
+----
+{
+  "persistent": {
+    "cluster.indices.close.enable": "false",
+    "indices.recovery.max_bytes_per_sec": "50mb",
+    ...
+  },
+  "transient": {
+  }
+}
+----
+// TESTRESPONSE[s/"50mb",/"50mb"/]
+// TESTRESPONSE[s/\.\.\.//]
+
+////
+[source,console]
+----
+PUT _cluster/settings
+{
+  "persistent" : {
+    "cluster.indices.close.enable": null,
+    "indices.recovery.max_bytes_per_sec": null
+  }
+}
+----
+// TEST[continued]
+////

docs/reference/cluster/update-settings.asciidoc

@@ -128,3 +128,5 @@ PUT /_cluster/settings
   }
 }
 --------------------------------------------------
+
+include::transient-settings-migration-guide.asciidoc[]

docs/reference/migration/index.asciidoc

@@ -1,40 +1,6 @@
 include::migration_intro.asciidoc[]
 
-* <<migrating-8.17,Migrating to 8.17>>
-* <<migrating-8.16,Migrating to 8.16>>
-* <<migrating-8.15,Migrating to 8.15>>
-* <<migrating-8.14,Migrating to 8.14>>
-* <<migrating-8.13,Migrating to 8.13>>
-* <<migrating-8.12,Migrating to 8.12>>
-* <<migrating-8.11,Migrating to 8.11>>
-* <<migrating-8.10,Migrating to 8.10>>
-* <<migrating-8.9,Migrating to 8.9>>
-* <<migrating-8.8,Migrating to 8.8>>
-* <<migrating-8.7,Migrating to 8.7>>
-* <<migrating-8.6,Migrating to 8.6>>
-* <<migrating-8.5,Migrating to 8.5>>
-* <<migrating-8.4,Migrating to 8.4>>
-* <<migrating-8.3,Migrating to 8.3>>
-* <<migrating-8.2,Migrating to 8.2>>
-* <<migrating-8.1,Migrating to 8.1>>
-* <<migrating-8.0,Migrating to 8.0>>
+* <<migrating-9.0,Migrating to 9.0>>
 
-include::migrate_8_17.asciidoc[]
-include::migrate_8_16.asciidoc[]
-include::migrate_8_15.asciidoc[]
-include::migrate_8_14.asciidoc[]
-include::migrate_8_13.asciidoc[]
-include::migrate_8_12.asciidoc[]
-include::migrate_8_11.asciidoc[]
-include::migrate_8_10.asciidoc[]
-include::migrate_8_9.asciidoc[]
-include::migrate_8_8.asciidoc[]
-include::migrate_8_7.asciidoc[]
-include::migrate_8_6.asciidoc[]
-include::migrate_8_5.asciidoc[]
-include::migrate_8_4.asciidoc[]
-include::migrate_8_3.asciidoc[]
-include::migrate_8_2.asciidoc[]
-include::migrate_8_1.asciidoc[]
-include::migrate_8_0.asciidoc[]
+include::migrate_9_0.asciidoc[]