Change docs and example for RandomSort

Clarify that input is a single node; change example to comply (and to
not use value-of, which doesn’t make sense when a node-set is returned)

I'd like to add a note about the fact that *properties* of the parent node are part of the randomization too (will probably trip you up if you expected only child documents to be returned) - but I'm not entirely sure how it should be written...