Paolo, I think you misunderstood - the form in question belongs to the *admin-panel* edit screen which necessarily always contains a form. Don't worry about that and please try to follow the instructions as given. If implemented correctly, you should find that if the description field is left empty while hitting 'save', Couch will automatically fill it with your custom text.
Please do try it and let me now if you have any difficulty in the process.
Please do try it and let me now if you have any difficulty in the process.