When I first started coding in Eclipse, it was really a challenge for me to add Javadoc to a Jar file.
First find the Javadoc page of the Jar file. In my case, I am looking for the Javadoc files of GraphStream API. It is at http://graphstream.sourceforge.net/javadoc/index.html.

Do that: Go to Package explorer

Right click on the Jar file, and choose Properties. On the new screen you will see Javadoc Location menu.

In the Javadoc location menu, enter the Javadoc URL. Now novice users make a mistake at that point. Javadoc is at http://graphstream.sourceforge.net/javadoc/index.html, but you should omit /index.html. In general, javadoc location ends with www…./doc or www…./javadoc. Click the Validate button, and if everything is ok, you should see this.