nuxeo-document-task-assignment-popup should return comment

In th element nuxeo-document-task-assignment-popup, [[comment]] is currently an input.
But comment is an output parameter.

This change enables to retrieve the comment for the task delegation in the activity of the platform.
Otherwise, comment entered by the user is ignored

Fix:

<nuxeo-textarea id="commentText" label="[[i18n('tasks.assignment.comment')]]" placeholder="[[i18n('tasks.assignment.placeholder')]]" value="{{comment}}" max-rows="4">
</nuxeo-textarea>