1. Open Windows -> Preferences.

　　2.Select jre from Installed JREs then hit Edit botton on the right.

　　3.browse  the installed jre path for the jre home.

　　4.Select xx.rt.jar item of the ‘JRE system libraries‘ and hit ‘Javadoc Location‘.

　　5.Select ‘Javadoc in archive‘ radiobutton,select the ‘External file‘ item,browse for apidocs.zip file for Archive path and set Path within archive as shown above.

ref:How to use apidocs.zip?