/**
     * Some connectors may want to pre-generate additional documents when {@link Connector#getDocumentById(String)} is called. In
     * such a case, the connector can use this method to obtain a writer for an additional document and use it in much the same
     * was as {@link Connector#newDocument(String)}. The resulting additional document will be included automatically when the
     * Connector returns the {@link #document() top-level document}.
     * 
     * @param id the identifier of the additional document; may not be null
     * @return the writer for the additional document; never null
     */
    DocumentWriter writeAdditionalDocument( String id );