Amend the Util#fixNull() Javadoc

core/src/main/java/hudson/Util.java

@@ -1199,7 +1199,8 @@ public class Util {
     }
 
     /**
-     * Convert {@code null} to a default <b>immutable</b> value.
+     * Convert {@code null} to a default value.
+     * @param defaultValue Default value. It may be immutable or not, depending on the implementation.
      * @since TODO
      */
     @Nonnull