I have a Java EE web page and web.xml descriptor of the project has not a interactive way to add information like servlet-servlet mapping, etc.
If you're using Servlets 3.0 in Java EE 6 than you can define servlet url's, filters, security, etc. directly in your code using annotations like @WebServlet, @WebFilter etc.
In Eclipse, if you add the XML Schema definition to the web.xml file than eclipse will tell you what elements are allowed in the place you're editing (after you hit ctrl + space).