[Plugin][Doc] CSVHook: documentation of arrayOnRow

[JusteRaimbault]

The behavior of the arrayOnRow parameter is not totally clear in the documentation of CSVHook: the flattening is done line by line and the consistency of the csv in terms of column numbers is not checked; in case of variable length arrays, it is necessary to set it to false to have consistent csv (and the arrays are written as strings). It differs from a former behavior where lines were duplicated when set to false if I remember well - the documentation should be more explicit I think.