First, you need to go to:-
project properties -> Java Build Path -> Libraries -> Selenium-java-2.42.2.jar
Then select the Selenium-java-2.xx.x.jar file & click to Expand.
Here you will see that the Javadoc location option is present. Select this option and click on Edit button.
On the next window add http://seleniumhq.github.io/selenium/docs/api/java/ link in javadoc location path textbox and click Ok.
Hope this will help you.