The node is often calculated and the signature should allow it to avoid static 
typing issues. Proposed rewording:

fn:idref($arg as xs:string*) as node()*
fn:idref($arg as xs:string*, $node as node()?) as node()*
Summary: Returns the sequence of element or attribute nodes with an IDREF 
value matching the value of one or more of the ID values supplied in $arg.

The function returns a sequence, in document order with duplicates eliminated, 
containing every element or attribute node $N that satisfies all the following 
conditions:

1new. If $node is the empty sequence, it returns the empty sequence.
...