[DOCS] Changed to use transient setting to reenabled allocation. Closes elastic#27677

Author: debadair

docs/reference/upgrade/cluster_restart.asciidoc

@@ -94,13 +94,19 @@ reenable allocation.
 ------------------------------------------------------
 PUT _cluster/settings
 {
-  "persistent": {
+  "transient": {
     "cluster.routing.allocation.enable": "all"
   }
 }
 ------------------------------------------------------
 // CONSOLE
 
+NOTE: Because <<cluster-update-settings.html#_precedence_of_settings, transient
+settings take precedence over persistent settings>>, this overrides the
+persistent setting used to disable shard allocation in the first step. If you
+don't explicitly reenable shard allocation after a full cluster restart, the
+persistent setting is used and shard allocation remains disabled.
+
 Once allocation is reenabled, the cluster starts allocating replica shards to
 the data nodes. At this point it is safe to resume indexing and searching,
 but your cluster will recover more quickly if you can wait until all primary

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -69,6 +69,12 @@ GET _cat/nodes
 +
 --
 
+NOTE: Because <<cluster-update-settings.html#_precedence_of_settings, transient
+settings take precedence over persistent settings>>, this overrides the
+persistent setting used to disable shard allocation in the first step. If you
+don't explicitly reenable shard allocation after a full cluster restart, the
+persistent setting is used and shard allocation remains disabled.
+
 Once the node has joined the cluster, reenable shard allocation to start using
 the node:
 
@@ -156,4 +162,4 @@ In the unlikely case of a network malfunction during the upgrade process that
 isolates all remaining old nodes from the cluster, you must take the
 old nodes offline and upgrade them to enable them to join the cluster.
 
-====================================================
+====================================================