Only moderators can edit revision history comments, including the comment for a rollback.
If you'd like to rollback with a custom comment, go into the revision history of the post and use the edit link on a specific revision:
This will bring you to the standard edit page (in which you can enter a custom comment), but containing the contents of the revision you chose instead of the current revision—so if you make no further changes, you are effectively performing a rollback.
If you've already made an edit and you want to add an important piece of information, you can do so in the comments. You may want to leave the message both in the revision history and as a comment if you think it's important that the user whose post you're editing sees your comment.
(As an aside, the moderator revision history editing tool is meant mostly for redacting offensive or sensitive information. You can use a custom moderator flag in that case, but for this situation, it'd be better to just leave a comment.)