Docs: Tweak upgrade docs

Tweak the upgrade instructions for moving from pre-6.3-with-x-pack to
post-6.3-default distribution. Specifically, you have to remove the
x-pack plugin before upgrading because 6.4 doesn't understand how to
remove it.

Relates to #34307

docs/reference/upgrade/cluster_restart.asciidoc

@@ -38,6 +38,10 @@ include::shut-down-node.asciidoc[]
 . *Upgrade all nodes.*
 +
 --
+include::remove-xpack.asciidoc[]
+--
++
+--
 include::upgrade-node.asciidoc[]
 include::set-paths-tip.asciidoc[]
 --
@@ -47,8 +51,6 @@ 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.*
 +

docs/reference/upgrade/remove-xpack.asciidoc

@@ -1,4 +1,8 @@
-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.
+IMPORTANT: If you are upgrading from a version prior to 6.3 and use {xpack}
+then you must remove the {xpack} plugin before upgrading with
+`bin/elasticsearch-plugin remove x-pack`. As of 6.3, {xpack} is included in
+the default distribution so make sure to upgrade to that one. If you upgrade
+without removing the {xpack} plugin first the node will fail to start. If you
+did not remove the {xpack} plugin and the node fails to start then you must
+downgrade to your previous version, remove {xpack}, and then upgrade again.
+In general downgrading is not supported but in this particular case it is.

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -44,6 +44,10 @@ include::shut-down-node.asciidoc[]
 . *Upgrade the node you shut down.*
 +
 --
+include::remove-xpack.asciidoc[]
+--
++
+--
 include::upgrade-node.asciidoc[]
 include::set-paths-tip.asciidoc[]
 --
@@ -53,8 +57,6 @@ 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.*
 +