If I've understood your question, I'd split the information into two: start with a graphic to illustrate the DTD or other structure, then use a definition list to present information about each element.

• http://www.altova.com/xmlspy/xml-schema-editor.html shows a fairly widespread presentation style
• this example from the DITA specifications illustrates truly thorough documentation of a large schema … you might not need to go that far :-}