Document unused type parameter for tests

src/test/java/org/apache/commons/lang3/test/NotVisibleExceptionFactory.java

@@ -45,7 +45,7 @@ public class NotVisibleExceptionFactory {
     }
 
     @Override
-    public Throwable getCause() {
+    public synchronized Throwable getCause() {
       return cause;
     }
   }