make transient assistant more clear on server events

fern/calls/call-outbound.mdx

@@ -1,3 +1,9 @@
+---
+title: Outbound Calling
+subtitle: Learn how to send outbound calls from Vapi.
+slug: calls/outbound-calling
+---
+
 ## Introduction to Outbound Calling
 
 Vapi’s outbound calling API lets you programmatically initiate single or batch calls to any phone number. You can schedule calls for specific dates and times, ideal for time-sensitive communications. Easily integrate outbound calling into your app for appointment reminders, automated surveys, and call campaigns.

fern/server-url/events.mdx

@@ -101,7 +101,15 @@ For inbound phone calls, you may want to specify the assistant based on the call
 }
 ```
 
-Your server should respond with a JSON object containing the assistant, like so:
+If you want to use an existing saved assistant instead of creating a transient assistant for each request, you can respond with the assistant's ID:
+
+```json
+{
+  "assistantId": "your-saved-assistant-id"
+}
+```
+
+Alternatively, if you prefer to define a transient assistant dynamically, your server should respond with the [assistant](/api-reference/webhooks/server-message#response.body.messageResponse.Server%20Message%20Response%20Assistant%20Request.assistant) object directly:
 
 ```json
 {
@@ -121,6 +129,8 @@ Your server should respond with a JSON object containing the assistant, like so:
 }
 ```
 
+
+
 If you'd like to play an error message instead, you can respond with:
 
 ```json