docs: call out current-state limitations for function.parameters on handoff tool

- Flags that function.parameters is API-only today (dashboard UX for the
  Tools page shipping in VapiAI/vapi#10727, squad-builder backend gap tracked
  in PRISM-471)
- Directs customers to put the handoff tool on the assistant's model.tools[]
  via the API instead of the squad-member Handoff Tools path until the
  backend gap (callAssistantsGet synthesizing without `function`) is closed

Author: dhruva-reddy

fern/squads/passing-data-between-assistants.mdx

@@ -20,6 +20,16 @@ This page is a decision guide. For end-to-end configuration of the handoff itsel
 
 Define `function.parameters` on the handoff tool. The LLM that's already generating the handoff tool call also fills in your custom arguments as part of the same call — no extra round-trip.
 
+<Warning>
+**Availability today:**
+
+- **API:** Fully supported. Send the JSON below via `POST /tool` or as part of your assistant's `model.tools[]` via `POST /assistant` / `PATCH /assistant`.
+- **Dashboard — Tools page:** UX for defining `function.parameters` on a handoff tool is shipping soon. Use the API in the meantime.
+- **Dashboard — Squad builder:** Configuring a handoff via the squad member's **Handoff Tools** section does NOT currently carry `function.parameters` through to the runtime tool (backend synthesizes the tool without the `function` field). Until that's fixed, put the handoff tool directly on the assistant's `model.tools[]` (via the API or the Tools page) instead of defining it per squad-member destination.
+</Warning>
+
+
+
 ```json
 {
   "type": "handoff",