The example given in the spec is enough for RELAX NG:
<?xml-model href="http://www.docbook.org/xml/5.0/rng/docbook.rng" schematypens="http://relaxng.org/ns/structure/1.0"?>
Or, if you are using the compact syntax:
<?xml-model href="whatever.rnc" type="application/relax-ng-compact-syntax"?>
oXygen XML does support this specification.