When I use quick documentaion lookup (Ctrl+Q) on j2ee classes or annotations in IDEA I only get an empty javadoc. It only contains the basics like class name.
How do I add the javadoc to the libs IDEA provides itself?
You can attach javadoc to any library you have configure in your module or project. Just access the project structure windows (File -> Project Structure), then select "modules" and select the module that has the dependency you want to configure.
Then select the "Dependencies" tab, select the dependency that's missing the javadoc and click "Edit". In the window that just showed up you see two buttons "Add" and "Specify Javadoc URL". If you have the javadoc in a jar file select the first one, if you want to point to a web site that contains the javadoc select the latest. That's it.