Updated warm-transfer with assistant doc (#605)

Author: ramsrib

fern/calls/assistant-based-warm-transfer.mdx

@@ -47,7 +47,7 @@ The `function.name` property identifies your transfer tool. Use this name when i
   "destinations": [
     {
       "type": "number",
-      "number": "+16509606568",
+      "number": "+14155551234",
       "message": "I am forwarding your call to customer support. Please stay on the line.",
       "transferPlan": {
         "mode": "warm-transfer-experimental",
@@ -67,6 +67,12 @@ The `function.name` property identifies your transfer tool. Use this name when i
         }
       }
     }
+  ],
+  "messages": [
+    {
+      "type": "request-start",
+      "content": "I am forwarding your call to customer support. Please stay on the line."
+    }
   ]
 }
 ```
@@ -151,6 +157,12 @@ The transfer assistant can be configured to handle various operator responses:
         }
       }
     }
+  ],
+  "messages": [
+    {
+      "type": "request-start",
+      "content": "Transferring you to our specialist. Please hold."
+    }
   ]
 }
 ```
@@ -174,8 +186,10 @@ Configure different transfer assistants for different departments:
       "description": "Sales Department",
       "transferPlan": {
         "mode": "warm-transfer-experimental",
+        "holdAudioUrl": "https://assets.example.com/hold-music.mp3",
         "fallbackPlan": {
-          "message": "Sorry, none of our account executives are available right now. Our team get back to you later"
+          "message": "Sorry, none of our account executives are available right now. Our team get back to you later",
+          "endCallEnabled": false
         },
         "transferAssistant": {
           "firstMessage": "Hey there, I have a potential customer interested in our enterprise plans. Are you available to pick up the call?",
@@ -199,6 +213,7 @@ Configure different transfer assistants for different departments:
       "description": "Technical Support",
       "transferPlan": {
         "mode": "warm-transfer-experimental",
+        "holdAudioUrl": "https://assets.example.com/hold-music.mp3",
         "transferAssistant": {
           "firstMessage": "Hey there, I have a customer experiencing issues with API integration. Can you help?",
           "maxDurationSeconds": 90,