Reference the new setting

from the migrate to data tiers docs

Author: joegallo

docs/reference/data-management/migrate-index-allocation-filters.asciidoc

@@ -56,6 +56,8 @@ To switch to using node roles:
 settings>> from your {ilm} policy.
 . <<stop-setting-custom-hot-attribute, Stop setting the custom hot attribute>>
 on new indices.
+. <<enforce-default-tier-preference, Enforce a default tier preference>>
+on new indices.
 . Update existing indices to <<set-tier-preference, set a tier preference>>.
 
 
@@ -136,6 +138,26 @@ DELETE _template/.cloud-hot-warm-allocation-0
 
 If you're using a custom index template, update it to remove the <<shard-allocation-filtering, attribute-based allocation filters>> you used to assign new indices to the hot tier.
 
+[discrete]
+[[enforce-default-tier-preference]]
+==== Enforce a default tier preference on new indices
+
+Setting <<enforcing-a-default-tier-preference,`cluster.routing.allocation.enforce_default_tier_preference`>> to `true`
+ensures that newly created indices will have a <<tier-preference-allocation-filter,_tier_preference>>, overriding the
+<<indices-create-index,create index>> API or an associated <<index-templates,index template>>, if either of those
+specifies `null` for the setting.
+
+[source,console]
+----
+PUT _cluster/_settings
+{
+  "persistent": {
+    "cluster.routing.allocation.enforce_default_tier_preference": true
+  }
+}
+----
+// TEST[continued]
+
 [discrete]
 [[set-tier-preference]]
 ==== Set a tier preference for existing indices

docs/reference/ilm/apis/migrate-to-data-tiers.asciidoc

@@ -20,6 +20,7 @@ in the <<migrate-index-allocation-filters, migration guide>>:
 
 . <<stop-setting-custom-hot-attribute, Stop setting the custom hot attribute on new indices>>
 . <<remove-custom-allocation-settings, Remove custom allocation settings from existing {ilm-init} policies>>
+. <<enforce-default-tier-preference, Enforce a default tier preference on new indices>>
 . <<set-tier-preference, Replace custom allocation settings from existing indices>> with the corresponding <<tier-preference-allocation-filter,tier preference>>
 
 [[ilm-migrate-to-data-tiers-request]]