Hi Luca,
If you haven't delivered the change set yet, you can do a compare against the main stream. After you deliver the change set, you cannot see the contents of the change set. This is a known requirement, see Plan Item 108881.

As of 602, the artifact/module history does indicate when a change happened and the change set, although it doesn't indicate what the change was.
Some workarounds that might help:

• Link the affected requirements directly to the same WI as the change set. You'll at least know what all was touched by the change set. You still won't know the details of the changes.
  
• Take a baseline before delivering the change set. You can then compare the before and after to see what changed. Optionally take a baseline after delivery also - although if you always baseline before delivering a change, the "after" baseline should be redundant.
• You can use RPE to generate a Word doc for the change set configuration (before you deliver) and the main stream, and use Word's compare/merge to do a "diff" (see this blog post). You could then attach that Word doc to your work item. This would also work if you were taking baselines as described above; you could compare the "before" baseline to the "after" state.