XQUF: Multiple target nodes

[ChristianGruen]

Various expressions of XQUF require targets to be single nodes. For example, the following queries are invalid:

replace node //multiple-results with <b/>
insert node <a/> into //multiple-results
rename node //multiple-results as 'b'

This restriction should be lifted to make FLWOR/simple map expressions superfluous:

for $a in //a return replace node $a with <b/>
//a ! (replace node . with <b/>)

→ replace node //a with <b/>

[ChristianGruen]

Documentation updated.