For any JAR file on the build path you can attach both source code and Javadoc to it as follows:
- right click on the project
- select Properties
- click on
Java Build Path
- select the Libraries tab
- open the library by pressing on the +
- click on Source attachment
- press Edit
- select the path (external file for JAR/ZIP files, external folder for directories, workspace for another Eclipse project)
- press OK
- click on Javadoc location
- press Edit
- fill in the Javadoc URL
- press OK
For SWT the Javadoc URL is
http://help.eclipse.org/stable/nftopic/org.eclipse.platform.doc.isv/reference/api/
Since this is an Eclipse issue I'm moving this to IDEs.