The Bel­gi­an data pro­tec­tion super­vi­so­ry aut­ho­ri­ty has published quite detail­ed expl­ana­ti­ons on the docu­men­ta­ti­on obli­ga­ti­on under Art. 30 GDPR. A sum­ma­ry in Eng­lish can be found on the web­site of the Field Fisher Law Firm.