A common property for providing additional information is rdfs:seeAlso . As spec says , you can create a seeAlso sub-property to convey more specific semantics. Regarding metadata encoding, I personally stick with Dublin Core , and I would add bNode so that the various properties of the external source can be recorded in a triple store.
So, if some_resource is what you have additional information about:
example:some_resource rdfs:seeAlso [ dcTerms:source "http://your.si.te/docs/foo.html"^^xsd:anyURI ; dcTerms:format "text/html" ; dcTerms:title "Foo user manual"@en ; dcTerms:language "en" ].
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