Converts a JSON-stat2 category-level note to a v1-style
value_comment object and indexes every data-frame row containing the
noted value.
pxweb_data_v2_value_comment(x, column_idx, value_code, comment)a pxweb_data_comment object.
a pxweb_data_v2 object.
integer index of the dimension column.
category value code that carries the note.
a normalized comment string.