docs: document say options and audio URL playback in Live Call Control

fern/calls/call-features.mdx

@@ -79,6 +79,23 @@ curl -X POST 'https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/742
 }'
 ```
 
+Optional fields:
+
+- `endCallAfterSpoken`: end the call after the message is spoken.
+- `interruptionsEnabled`: let the caller interrupt the message. Defaults to `false`.
+- `interruptAssistantEnabled`: stop any speech already in progress or queued and say this message immediately. Defaults to `false`.
+
+To play a pre-recorded audio file instead of synthesized speech, set `content` to the file's URL. The URL must be the entire `content` value and must use `https://`; any other content, including an `http://` URL or a URL inside a sentence, is spoken by the voice provider as text.
+
+```bash
+curl -X POST 'https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/control' \
+-H 'content-type: application/json' \
+--data-raw '{
+  "type": "say",
+  "content": "https://example.com/audio/hold-message.wav"
+}'
+```
+
 ### 2. Add Message to Conversation
 Adds a message to the conversation history and optionally triggers a response.