Return words in `vocab` ordered by saliency score.

def _words_by_salience_score(vocab, topic_word_distrib, doc_topic_distrib, doc_lengths, n=None, least_to_most=False):
    """Return words in `vocab` ordered by saliency score."""
    saliency = get_word_saliency(topic_word_distrib, doc_topic_distrib, doc_lengths)
    return _words_by_score(vocab, saliency, least_to_most=least_to_most, n=n)