Document that Gemini requires native or prompted output mode for structured output streaming (#3032)

Author: DouweM

docs/output.md

@@ -581,6 +581,10 @@ async def main():
 
 _(This example is complete, it can be run "as is" — you'll need to add `asyncio.run(main())` to run `main`)_
 
+As setting an `output_type` uses the [Tool Output](#tool-output) mode by default, this will only work if the model supports streaming tool arguments. For models that don't, like Gemini, try [Native Output](#native-output) or [Prompted Output](#prompted-output) instead.
+
+### Streaming Model Responses
+
 If you want fine-grained control of validation, you can use the following pattern to get the entire partial [`ModelResponse`][pydantic_ai.messages.ModelResponse]:
 
 ```python {title="streamed_user_profile.py" line_length="120"}