fix(docs): describe fallback rest option

src/v1beta1/gateway_service_client.ts

@@ -64,7 +64,7 @@ export class GatewayServiceClient {
    *
    * @param {object} [options] - The configuration object.
    * The options accepted by the constructor are described in detail
-   * in [this document](https://github.com/googleapis/gax-nodejs/blob/master/client-libraries.md#creating-the-client-instance).
+   * in [this document](https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#creating-the-client-instance).
    * The common options are:
    * @param {object} [options.credentials] - Credentials object.
    * @param {string} [options.credentials.client_email]
@@ -87,11 +87,10 @@ export class GatewayServiceClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean} [options.fallback] - Use HTTP fallback mode.
-   *     In fallback mode, a special browser-compatible transport implementation is used
-   *     instead of gRPC transport. In browser context (if the `window` object is defined)
-   *     the fallback mode is enabled automatically; set `options.fallback` to `false`
-   *     if you need to override this behavior.
+   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
+   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   *     For more information, please check the
+   *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    */
   constructor(opts?: ClientOptions) {
     // Ensure that options include all the required fields.