Added missing javadocs. Created a separate class for message subjects used by distribued device store
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortStatusEvent.java b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortStatusEvent.java
index 822ba7a..0bdfdbf 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortStatusEvent.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortStatusEvent.java
@@ -5,6 +5,10 @@
 import org.onlab.onos.net.provider.ProviderId;
 import org.onlab.onos.store.common.impl.Timestamped;
 
+/**
+ * Information published by GossipDeviceStore to notify peers of a port
+ * status change event.
+ */
 public class InternalPortStatusEvent {
 
     private final ProviderId providerId;