Poking around in the 8.5 Devel Documentation section 8.13.1, the XML Type, I noticed that:
"The xml type does not validate input values against a document type declaration (DTD), even when the input value specifies a DTD"
I did not notice any mention of validating against XML schema. Now that it is 2010, the XML schema 1.1 spec is in draft form of one sort or another, is it safe to leave out mentioning validating against XML schema in the PostGreSQL documentation. That is, PostGreSQL users should know enough about XML that if you can't validate against a DTD, why should you be able to validate against XML schema?
I would tent towards redundancy, and include the XML schema part.