¿Anybody knows on which part of XWorkplace can be added the description for this classes?
The descriptions are in <Xworkplace-path>\help\xfcls001.txt (or substitute appropriate language code). Anyone with write access to XWP CVS should be able to make the same change (for everyone) to the file in <cvs-root>\001\misc.
Incidentally, the XWP class list is actually compiled into eWorkplace, it's just not registered or created in a normal install. With a little REXX knowledge it's possible to register the class and create the object under eCS. (Although in this case the file xfcls001.txt isn't included at all, so if you want descriptions you'd need to copy it from elsewhere.)