Leonid,
Some more ideas:
You can also open the Javadoc view using Window > Show View > Other > Java > Javadoc which will show Javadoc for the method / class currently in focus.
To attach javadoc to a JAR, simply bring up the context menu for that JAR in the package explorer and choose Properties.Go to the Javadoc node and specify the location of the Javadoc, either online or local.
Hope this helps.