Added a brief comment to explain SuppressWarnings statement (thanks britter).

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1672242 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/lang3/SerializationUtils.java

@@ -121,7 +121,7 @@ public class SerializationUtils {
      * @return the serialized and deseralized object
      * @since 3.3
      */
-    @SuppressWarnings("unchecked")
+    @SuppressWarnings("unchecked") // OK, because we serialized a type `T`
     public static <T extends Serializable> T roundtrip(final T msg) {
         return (T) SerializationUtils.deserialize(SerializationUtils.serialize(msg));
     }