Add text inside comment box in Twenty Eleven Theme

I’d like to add text inside the comment box that says something like “We’ll delete your comment if it’s mean. Here’s a link to our Terms of Use.” I have been playing around with the comments.php file (child theme) and I can get the text to show up above or below the comments box, which would be fine except that it also shows up on my Pages, which do not allow comments.
How can I get my text to show up ONLY on blog posts that allow comments?