code: public void updateDocument(String accountId, String envelopeId, String documentId) throws ApiException {
      updateDocument(accountId, envelopeId, documentId, null);
    }
docs: Adds a document to an existing draft envelope.
  Adds a document to an existing draft envelope.
  @param accountId The external account number (int) or account ID Guid. (required)
  @param envelopeId The envelopeId Guid of the envelope being accessed. (required)
  @param documentId The ID of the document being accessed. (required)
  @return void