Add comments on when javac will generate `$closeResource`

Change-Id: I11e1bd3d752d394b6df902debf554bad5d391cc3
diff --git a/src/test/examplesJava9/twraddsuppressed/TestClass.java b/src/test/examplesJava9/twraddsuppressed/TestClass.java
index 61285ce..fcf0d6f 100644
--- a/src/test/examplesJava9/twraddsuppressed/TestClass.java
+++ b/src/test/examplesJava9/twraddsuppressed/TestClass.java
@@ -20,6 +20,8 @@
   }
 
   public static void bar() {
+    // NOTE: The $closeResource helper is _only_ generated by the JDK-9 compiler.
+    //
     // Use twr twice to have javac generate a shared $closeResource helper.
     try (MyClosable closable = new MyClosable()) {
       foo();