Replace 1.5 with the version you want (if refactoring has taken place you get a HTTP 404 tough) and then hit the small "Frames" link in the top center.
But why do you need that? First gwt-user.jar contains source files so you should have JavaDoc for GWT classes right in your IDE. If not then you have probably gwt-servlet.jar on classpath as well. Simply remove it or make sure that gwt-user.jar is before gwt-servlet.jar in your classpath.
Also you have the full GWT SDK installed via Eclipse that can be found in Eclipse/plugins/com.google.gwt.eclipse.sdkbundle_<version>. It contains the full JavaDoc of the GWT version you use.