Doc: Add bundled JDK info (#12369)

Document options and impacts of bundled JDK

docs/static/jvm.asciidoc

@@ -8,13 +8,25 @@
 * Java 11
 * Java 14
 
-See the https://www.elastic.co/support/matrix#matrix_jvm[Elastic Support Matrix]
-for the official word on supported versions across releases.
-
 Use the
 http://www.oracle.com/technetwork/java/javase/downloads/index.html[official
 Oracle distribution] or an open-source distribution, such as
 http://openjdk.java.net/[OpenJDK].
+See the https://www.elastic.co/support/matrix#matrix_jvm[Elastic Support Matrix]
+for the official word on supported versions across releases.
+
+.Bundled JDK
+[NOTE]
+===== 
+{ls} offers architecture-specific
+https://staging-website.elastic.co/downloads/logstash[downloads] that include
+AdoptOpenJDK 11, the latest long term support (LTS) release of JDK.
+
+Use the JAVA_HOME environment variable if you want to use a JDK other than the
+version that is bundled. 
+If you have the JAVA_HOME environment variable set to use a custom JDK, Logstash
+will continue to use the JDK version you have specified, even after you upgrade. 
+=====
 
 [float]
 [[check-jvm]]