Added documentation on the merge id

    constexpr const static char serialize_end = '\0';

    size_t size_;

    /*

     * ID of the merge operation that this operand belongs to.

     * This ID is used only in append operations to communicate the starting

     * write offset from the asynchronous Merge operation back to the caller in

     * `increase_size_impl()`.

     */

    uint16_t merge_id_;

    bool append_;