Added documentation on the return value of the start function (#34)

Author: MattPoetker

fern/sdk/web.mdx

@@ -25,6 +25,13 @@ You can start a web call by calling the `.start()` function. The `start` functio
 1. **a string**, representing an assistant ID
 2. **an object**, representing a set of assistant configs (see [Create Assistant](/api-reference/assistants/create-assistant))
 
+The `start` function returns a promise that resolves to a call object. For example:
+
+```javascript
+const call = await vapi.start(assistantId);
+// { "id": "bd2184a1-bdea-4d4f-9503-b09ca8b185e6", "orgId": "6da6841c-0fca-4604-8941-3d5d65f43a17", "createdAt": "2024-11-13T19:20:24.606Z", "updatedAt": "2024-11-13T19:20:24.606Z", "type": "webCall", ... }
+```
+
 #### Passing an Assistant ID
 
 If you already have an assistant that you created (either via [the Dashboard](/quickstart/dashboard) or [the API](/api-reference/assistants/create-assistant)), you can start the call with the assistant's ID: