[F&O] 2.5 fn:document-uri

1) There is a cross-reference to DM section 5.1. But this section does 
not contain a description of the document-uri accessor.

2) As fn:document-uri returns () if the document-uri property is a 
relative URI, the last sentence should be:
If the fn:document-uri($arg) does not return the empty sequence, then 
the following expression always holds.

Till Westmann