I just had to dig through this issue myself and succeeded. Contrary to what others have offered as solutions, the path to my happy ending was directly correlated to JavaDoc. No "src.zip" files necessary. My trials and tribulations in the process involved finding the CORRECT JavaDoc to point at. Pointing a Java 1.7 project at Java 8 Javadoc does NOT work. (Even if "jre8" appears to be the only installed JRE available.) Thus, I beat my head against the brick wall unnecessarily.
Window > Preferences > Java > Installed JREs
If the JRE of your project is not listed (as happened to me when I migrated a jre7 project to a new jre8 workspace), you will need to add it here. Click "Add..." and point your Workspace at the desired jre folder. (Mine was C://Program Files/Java/jre7). Then "Edit..." the now-available JRE, select the rt.jar, and click "Javadoc Location..." and aim it at the correct javadoc location. For my use:
For jre7 -- http://docs.oracle.com/javase/7/docs/api/ For jre8 -- http://docs.oracle.com/javase/8/docs/api/
Voila, hover tooltip javadoc is re-enabled. I hope this helps anyone else trying to figure this problem out.