Hi Bartosz,
Is it possible to enclose result in one element <mark>hello world</mark> ?
unfortunately this is not possible, because at the stage of marking the terms, we don’t have any information if the matched terms were the result of a single or phrase search. If all of your adjacent query terms will be the result of phrase queries, you could rewrite your result with some additional XQuery code (using e.g. a transform expression [1]).
Hope this helps, Christian