I forgot to mention how to enable this functionality and how to get these descriptions later.
To enable discription entering ability you need to specify 'description_field' => 'DescriptionFieldName' in particular field declaration in unit config.
Of course you need to create a real or virtual field that will store all that descriptions.
Descriptions are stored the same way as files are - "descr1|descr2|....". Positions of description and uploaded file in 2 fields (with files and with descriptions) matches, e.g. file at position 2 in file field will have description at position 2 in description field.