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

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

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

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