Found it, I believe : for you who may have the same problem :

Sales => Quotation => on the table that lists the products that are included in the quotation, you have a parameter  icon (head of the table, at the right,  next to sub-total in my case). Verify there that "description" is enabled"
It was not on my side and was preventing me to add nice notes / sections

All the best to you, Luc