In general this is the task instance that you are about to work on. So you should have access to all its data. Look at the Task class and you'll notice the relationship. Just so you know it uses expression language to access data instead of getters so that's you it's: task.descriptions instead of task.getDescriptions.
Then when you look at what HT work item handler does you'll notice that it is responsible for filling in such information - put comment into descriptions field.
Thanks Maciej - that's very useful.
This (an the way in which "Content" seems to override other task input mappings) is an area where the documentation isn't very clear. Should I submit a Jira suggesting that some clarification be added, or is this an area that is about the change in jBPM6?
yes, please do create jira for it. Soon documentation will be enhanced so the more input on areas to focus the better.