Fix CSV export of documents description

The "description" widget is not configured correctly, it should be using "texarea" instead of "text".

Steps to reproduce:

• fill the description on a document
• in the "advanced search" form, add criteria for this document to be listed in results
• also select the "description" column as it's not displayed by default

=> see that the formatting is lost when displaying the description in results (end of lines, mostly)