private void setSignificanceAndDocumentFrequency(TaggableItem target, BlockItem original) {
        if (hasExplicitSignificance(original)) target.setSignificance(getSignificance(original)); //copy
        var documentFrequency = getDocumentFrequency(original);
        if (documentFrequency.isPresent()) target.setDocumentFrequency(documentFrequency.get());
    }
Replaces query terms with their stems

@author Mathias Lidal
@author bratseth
@author Steinar Knutsen
container-search/src/main/java/com/yahoo/prelude/querytransform/StemmingSearcher.java