Update JavaDoc for @DesignRoot. (#8805)

Emphasise that both location and filename for the design file can be specified in the value parameter.

server/src/main/java/com/vaadin/annotations/DesignRoot.java

@@ -26,10 +26,11 @@ import com.vaadin.ui.declarative.Design;
  * Marks the component as the root of a design (html) file.
  * <p>
  * Used together with {@link Design#read(com.vaadin.ui.Component)} to be able
- * the load the design without further configuration. The design is loaded from
- * the same package as the annotated class and by default the design filename is
- * derived from the class name. Using the {@link #value()} parameter you can
- * specify another design file name.
+ * the load the design without further configuration. By default, the design
+ * is loaded from the same package as the annotated class and the design
+ * filename is derived from the class name. You can override the default
+ * behaviour by using the {@link #value()} parameter in order to load the
+ * design from a different package or with a non-default filename.
  *
  * @since 7.4
  * @author Vaadin Ltd