If you want to add all the ticket comments, start by navigating to Admin > Invoice Preferences and make sure that "Do not include ticket details on the Invoice PDF" is unchecked.
Then head to Admin > Invoices > Templates and click the edit link for the 'Invoice Ticket Template'.
From here, you can adjust how you want the comments to appear:
And that's it! For a more definitive guide on using our new template system, check out this!