Most of the workflow variables are accesible in jBPM process definition in alfresco.
Such as bpm:assignee is available as bpm_assignee, bpm:workflowDueDate is available as bpm_workflowDueDate, bpm:workflowPriority as bpm_workflowPriority etc
But the task Comments entered by the user is unfortunately not available directly. However, you can still access it by the following syntax in the script.
var comment = "";
if (token.comments.size() > 0)
comment = token.comments.get(0).message;
The comment variable is now available in your script.