minor Kryo related cleanup

Change-Id: I772fc1f776563d30f1a4081c6fddd5739d985263
diff --git a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableSetSerializer.java b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableSetSerializer.java
index c08bf9a..051a843 100644
--- a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableSetSerializer.java
+++ b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/ImmutableSetSerializer.java
@@ -18,6 +18,9 @@
 
     private final CollectionSerializer serializer = new CollectionSerializer();
 
+    /**
+     * Creates {@link ImmutableSet} serializer instance.
+     */
     public ImmutableSetSerializer() {
         // non-null, immutable
         super(false, true);