[DOCS] Clarified that you must remove X-Pack plugin when upgrading from pre-6.3. (#32016)

Author: debadair

docs/reference/upgrade/cluster_restart.asciidoc

@@ -47,6 +47,8 @@ include::set-paths-tip.asciidoc[]
 Use the `elasticsearch-plugin` script to install the upgraded version of each
 installed Elasticsearch plugin. All plugins must be upgraded when you upgrade
 a node.
++
+include::remove-xpack.asciidoc[]
 
 . *Start each upgraded node.*
 +
@@ -91,7 +93,7 @@ already have local shard copies.
 +
 --
 When all nodes have joined the cluster and recovered their primary shards,
-reenable allocation by restoring `cluster.routing.allocation.enable` to its 
+reenable allocation by restoring `cluster.routing.allocation.enable` to its
 default:
 
 [source,js]

docs/reference/upgrade/remove-xpack.asciidoc

@@ -0,0 +1,4 @@
+IMPORTANT: If you use {xpack} and are upgrading from a version prior to 6.3,
+remove {xpack} before restarting: `bin/elasticsearch-plugin remove x-pack`. As
+of 6.3, {xpack} is included in the default distribution. The node will fail to
+start if the old {xpack} plugin is present.

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -53,6 +53,8 @@ include::set-paths-tip.asciidoc[]
 Use the `elasticsearch-plugin` script to install the upgraded version of each
 installed Elasticsearch plugin. All plugins must be upgraded when you upgrade
 a node.
++
+include::remove-xpack.asciidoc[]
 
 . *Start the upgraded node.*
 +
@@ -144,7 +146,7 @@ for each node that needs to be updated.
 
 --
 
-. *Restart machine learning jobs.* 
+. *Restart machine learning jobs.*
 
 [IMPORTANT]
 ====================================================