the short answer:
Drive.Comments.list(id)
the how to:
"Drive" is a call to the drive api but this is still experimental, in your script you need to go in "ressources > advanced google services" and activate the Drive API. You also need to follow the link at the end "These services must also be enabled in the Google Developers Console." and activate the drive API there.
here an example:
you can add a comment to this google document
and then have a look at this script webapp to retrieve all the comments.
the source code of the webapp is here (It's a little mess in there, because I used two differents methods to get the Drvie API to work)