'xref' is used to create cross references to other parts of the
document. It is an empty element - the cross reference text depends on
various attributes. Quoting 'DocBook: The Definitive Guide':
1. If the endterm attribute is specified on xref, the content of the
element pointed to by endterm will be used as the text of the
cross-reference.
2. Otherwise, if the object pointed to has a specified XRefLabel, the
content of that attribute will be used as the cross-reference text.
