* @param reader The supplier for DocumentReader.
     * @param docPerReq The number of documents to process per request.
     * @param waitController The runnable to control waiting between requests.
     * @return A Promise that will be resolved with the SuggestIndexResponse or rejected with an error.
     */
    public Deferred<SuggestIndexResponse>.Promise indexFromDocument(final Supplier<DocumentReader> reader, final int docPerReq,