If there is popular demand for another subject, xmlGeek might be sympathetic in the next revision. The semantics of each qualifier value are sometimes dependent on the title. These will be enumerated in detail for all titles, but in this first release only DTD will be documented. XML allows you to omit its use. Or not even to have one.
Opinions vary - and are summarised below.
Good DTDs are huge. No unpleasant surprises in document instances. DTDs are the contract between authors and readers. I can't use DTDs. I never write XML documents for which DTDs would be useful.