8276700: Improve java.lang.ref.Cleaner javadocs

Reviewed-by: rriggs, mchung

src/java.base/share/classes/java/lang/ref/Cleaner.java

@@ -86,9 +86,13 @@ import java.util.function.Function;
  * by the Cleaner when the CleaningExample instance has become phantom reachable.
  * <pre>{@code
  * public class CleaningExample implements AutoCloseable {
- *        // A cleaner, preferably one shared within a library
- *        private static final Cleaner cleaner = <cleaner>;
+ *        // A cleaner (preferably one shared within a library,
+          // but for the sake of example, a new one is created here)
+ *        private static final Cleaner cleaner = Cleaner.create();
  *
+ *        // State class captures information necessary for cleanup.
+ *        // It must hold no reference to the instance being cleaned
+ *        // and therefore it is a static inner class in this example.
  *        static class State implements Runnable {
  *
  *            State(...) {