code: public EnvelopeDocument updateDocument(String accountId, String templateId, String documentId, EnvelopeDefinition envelopeDefinition) throws ApiException {
      return updateDocument(accountId, templateId, documentId, envelopeDefinition, null);
    }
docs: Adds a document to a template document.
  Adds the specified document to an existing template document.
  @param accountId The external account number (int) or account ID Guid. (required)
  @param templateId The ID of the template being accessed. (required)
  @param documentId The ID of the document being accessed. (required)
  @param envelopeDefinition  (optional)
  @return EnvelopeDocument