Document the NamedNodeURIField

[timbl]

This field type is missing from the documentation.

It prompts the user foa URI, but, unlike a text field, the vale is a named node in the RDF graph. Sometimes users have to give the explicit URIs of things. Then best to have them in the data as a thing not as a string. Of course ideally they should never see explicit URIs, bu when they have to this field is useful.