diff --git a/lib/robot.js b/lib/robot.js
index 6dc691b2..ee5b0238 100644
--- a/lib/robot.js
+++ b/lib/robot.js
@@ -56,10 +56,11 @@ class Robot {
   }
 
   /**
-   * Authenticate as an installation and get a GitHub client that can be used
-   * to make API calls.
+   * Authenticate and get a GitHub client that can be used to make API calls.
    *
-   *  **Note**: `robot.auth` is asynchronous, so it needs to be prefixed with a [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) to wait for the magic to happen.
+   * **Note**: `robot.auth` is asynchronous, so it needs to be prefixed with a
+   * [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await)
+   * to wait for the magic to happen.
    *
    * @example
    *
@@ -69,7 +70,12 @@ class Robot {
    *    });
    *  };
    *
-   * @param {number} id - ID of the installation, which can be extracted from an event:
+   * @param {number} [id] - ID of the installation, which can be extracted from
+   * `event.payload.installation.id`. If called without this parameter, the
+   * client wil authenticate [as the integration](https://developer.github.com/early-access/integrations/authentication/#as-an-integration)
+   * instead of as a specific installation, which means it can only be used for
+   * [integration APIs](https://developer.github.com/v3/integrations/).
+   *
    * @returns {Promise<github>} - An authenticated GitHub API client
    */
   async auth(id) {