If some CDK class inherits JavaDoc from a class outside the classpath, it is not showing up in the output. This problem is known and outlined in this StackOverflow question too:
http://stackoverflow.com/questions/1243063/inheritdoc-not-inheriting-superclass-javadoc-in-eclipse
Right now, the signature code is suffering from the JavaDoc limitation.
I do not see a solution to this right now :(
Gilleans soluton, while a bit hacky would work - we coud set up a target in javadoc.xml to extract the sources for the external libs and incldue them in the source path of javadoc
Confirmed the issue in the current JavaDoc: http://cdk.github.io/cdk/1.5/docs/api/org/openscience/cdk/signature/AtomSignature.html