Docs: Add known issue for removing x-pack

6.4 can no longer remove the x-pack plugin so users must remove it
before upgrading to 6.4+. We've already updated the upgrade
documentation. This updates the release notes and breaking changes docs.

Closes #34307

Author: nik9000

docs/reference/migration/migrate_6_4.asciidoc

@@ -7,6 +7,16 @@
 This section discusses the changes that you need to be aware of when migrating
 your application to Elasticsearch 6.4.
 
+[IMPORTANT]
+.bin/plugin can no longer remove x-pack
+=========================================
+
+`bin/plugin remove x-pack` no longer works. You should remove the {xpack}
+plugin before upgrading to 6.4 or any later versions. More instructions
+available in <<rolling-upgrades>> and <<restart-upgrade>>.
+
+=========================================
+
 * <<breaking_64_api_changes>>
 * <<breaking_64_scripting_changes>>
 

docs/reference/release-notes/6.4.asciidoc

@@ -98,9 +98,18 @@ Caused by: org.elasticsearch.common.xcontent.XContentParseException: [-1:####] [
 
 ======================================================
 
+[IMPORTANT]
+.bin/plugin can no longer remove x-pack
+=========================================
+
+`bin/plugin remove x-pack` no longer works. You should remove the {xpack}
+plugin before upgrading to 6.4 or any later versions.
+
+=========================================
+
 [float]
 === Known issues
-See the important section above.
+See the important sections above.
 
 {es} 6.4.0 fails to start when PEM encoded private key files that have been exported from `PKCS#12`
 keystores are in use. These files can be identified by the existence of lines that start with either