I have downloaded Java API documentation from http://www.oracle.com/technetwork/java/javase/downloads/index.html#docs and have supposedly attached it to Eclipse using the >
Choose one class you want to view its documentation and press Ctrl+click over it, the Javadoc page will inform you that there is no Javadoc file attached and bellow will see a button named "Attach File". Press that button and browse to the directory where JDK is installed, normally for Win is C:\Program files\Java\jdk_xxx and inside this folder there is a src.zip file - sleect it and press OK and all is done - you already have Javadoc attached.