Adding text to Comment box heading

Twenty Eleven and Twenty Eleven Child

Just under the Leave a Reply heading above the Comments box is the following text:
Your email address will not be published. Required fields are marked *

How can I add text to this, possibly even adding a third line?

Searching my entire blog, I found:
Your email address will not be published.

but not:
Required fields are marked

in blog\wp-includes\comment-template.php

Just about every response recommends making changes to the child theme only.

Where and how can I add the additional text?

I want it to state:
Your email address will not be published. Required fields are marked *
Last Name is mandatory.
(not in italic)

Thanks,

Bob