Fixed JavaDoc for setting page title. (#12433)

server/src/main/java/com/vaadin/server/Page.java

@@ -1345,8 +1345,8 @@ public class Page implements Serializable {
      * Sets the page title. The page title is displayed by the browser e.g. as
      * the title of the browser window or as the title of the tab.
      * <p>
-     * If the title is set to null, it will not left as-is. Set to empty string
-     * to clear the title.
+     * If this value is set to null, the previously set page title will be left
+     * as-is. Set to empty string to clear the title.
      *
      * @param title
      *            the page title to set