This can be easily accomplished by adding a small snippet of code to your to your Question text.
1. If you aren't in Source view, click the "Source" button in the editor:
2. And copy and paste the following after your question text:
<script>
$(document).on('ready pjax:scriptcomplete',function(){
$('#question{QID} .text-item label').text('TEXT FOR COMMENT BOX:');
});
</script>
Replace the TEXT FOR COMMENT BOX with your own label text -- or delete that text (keep the single quotes) if you want to remove the label altogether.
