8014966: Add the proper Javadoc to @Contended

Summary: more extensive description.
Reviewed-by: dholmes, mduigou, martin

src/share/classes/sun/misc/Contended.java

@@ -31,7 +31,42 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 /**
- * This annotation marks classes and fields as considered to be contended.
+ * <p>An annotation expressing that objects and/or their fields are
+ * expected to encounter memory contention, generally in the form of
+ * "false sharing". This annotation serves as a hint that such objects
+ * and fields should reside in locations isolated from those of other
+ * objects or fields. Susceptibility to memory contention is a
+ * property of the intended usages of objects and fields, not their
+ * types or qualifiers. The effects of this annotation will nearly
+ * always add significant space overhead to objects. The use of
+ * {@code @Contended} is warranted only when the performance impact of
+ * this time/space tradeoff is intrinsically worthwhile; for example,
+ * in concurrent contexts in which each instance of the annotated
+ * class is often accessed by a different thread.
+ *
+ * <p>A {@code @Contended} field annotation may optionally include a
+ * <i>contention group</i> tag. A contention group defines a set of one
+ * or more fields that collectively must be isolated from all other
+ * contention groups. The fields in the same contention group may not be
+ * pairwise isolated. With no contention group tag (or with the default
+ * empty tag: "") each {@code @Contended} field resides in its own
+ * <i>distinct</i> and <i>anonymous</i> contention group.
+ *
+ * <p>When the annotation is used at the class level, the effect is
+ * equivalent to grouping all the declared fields not already having the
+ * {@code @Contended} annotation into the same anonymous group.
+ * With the class level annotation, implementations may choose different
+ * isolation techniques, such as isolating the entire object, rather than
+ * isolating distinct fields. A contention group tag has no meaning
+ * in a class level {@code @Contended} annotation, and is ignored.
+ *
+ * <p>The class level {@code @Contended} annotation is not inherited and has
+ * no effect on the fields declared in any sub-classes. The effects of all
+ * {@code @Contended} annotations, however, remain in force for all
+ * subclass instances, providing isolation of all the defined contention
+ * groups. Contention group tags are not inherited, and the same tag used
+ * in a superclass and subclass, represent distinct contention groups.
+ *
  * @since 1.8
  */
 @Retention(RetentionPolicy.RUNTIME)
@@ -39,7 +74,10 @@ import java.lang.annotation.Target;
 public @interface Contended {
 
     /**
-      Defines the contention group tag.
+     * The (optional) contention group tag.
+     * This tag is only meaningful for field level annotations.
+     *
+     * @return contention group tag.
      */
     String value() default "";
 }