Berkeley DB XML Reference Guide:

fn:doc($uri as xs:string?) as document-node()?

Returns a document using an xs:anyURI supplied as xs:string. See the specification for details.

$uri: A URI representing the document. For a URI that begins with "dbxml:/" or that uses a base URI of "dbxml:/" (the default) only the component following the last '/' in the URI is treated as the document name. Components of the URI before that are assumed to be the path to the container. This means that a document name in the "dbxml:" URI scheme cannot contain the character '/'. For example, the URI "dbxml:/a/b/c/doc" used as $uri refers to the document "doc" in the container found via the relative URI "a/b/c."
It is also possible to use a URI from another scheme, such as "http://website/path-to-document" or "file://path-to-document" if you have enabled external file access during construction of the XmlManager object.