Insert the function doc `cells` at their correct position and updates `pos_dict`.

def insert_cells(cells, pos_dict, ft_name, append=False):
    "Insert the function doc `cells` at their correct position and updates `pos_dict`."
    idx = get_insert_idx(pos_dict, ft_name)
    if append or idx == -1: cells += [get_doc_cell(ft_name), get_empty_cell()]
    else:
        cells.insert(idx, get_doc_cell(ft_name))
        cells.insert(idx+1, get_empty_cell())
        pos_dict = update_pos(pos_dict, ft_name, 2)
    return cells, pos_dict