At http://www.docbook.org/schemas/htmlforms, it says: "The HTML form elements are part of the standard DocBook V5.0 distribution". But that is not true. The DocBook 5.0 (and 5.1) schemas available at http://www.docbook.org/schemas/5x do not include the HTML forms elements.
The HTML forms elements are not documented in the printed TDG5 book (http://oreilly.com/catalog/9780596805012/), but they are covered in the online TDG5 (http://docbook.org/tdg5/en/html/docbook.html).
I think adding the HTML forms elements to DocBook: The Definitive Guide was a mistake. The elements have always been available as a separate customization layer.
Given the continuing evolution of the HTML specification, it seems imprudent to do more.
Sorry for the confusion.
The information at http://www.docbook.org/schemas/htmlforms should be updated.