In admin, Email Templates, click the language of the email you wish to edit. On the next screen, on the HTML Content tab, note that in the list of Available Macros, there is {$DATA.customer_comments}. Add this to the HTML of the email wherever you wish. For example:
Find:
<p><a href="{$DATA.link}">{$DATA.link}</a></p>
Add after:
<p>Your comments about this order: "{$DATA.customer_comments}"</p>