Available for ZWO file downloads for Zwift only. More systems coming soon. See in the image, it shows you exactly where your comment will appear in the interval. We will plan on adding interval comments to other platforms. I know GoldenCheetah has also said they will support importing .zwo files with interval comments soon.
If enough people request it we could add them to .erg files for TrainerRoad. Next on the roadmap is adding them to our iOS and Android training apps.
If you want to delete a row, just clear the comment out and it will automatically be removed when you save it.
If you have any questions, just let us know. I will create a video showing it soon.