Javadoc improvements
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java b/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
index 4c9eefa..ee8d9c1 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
@@ -2,6 +2,8 @@
 
 /**
  * Representation of a message subject.
+ * Cluster messages have associated subjects that dictate how they get handled
+ * on the receiving side.
  */
 public class MessageSubject {