The function returns true if and only if the function call fn:doc($uri)
would return a document node.
fn:doc-available( $uri as xs:string?xs:booleanIf $uri is an empty sequence, this function returns false.
If a call on fn:doc($uri) would return a document node, this function
returns true.
A dynamic error is raised if $uri is not
a valid URI according to the rules applied by the implementation of
fn:doc.
In all other cases this function returns false. This
includes the case where an invalid URI is supplied, and also the case where
a valid relative URI reference is supplied, and cannot be resolved,
for example because the static base URI is absent.
If this function returns true, then calling fn:doc($uri)
within the same must return a document node. However,
if nondeterministic processing has been selected for the fn:doc function,
this guarantee is lost.