fix StreamingResponse example

Fixes #14680

docs/en/docs/advanced/custom-response.md

@@ -217,7 +217,15 @@ You can also use the `status_code` parameter combined with the `response_class`
 
 Takes an async generator or a normal generator/iterator and streams the response body.
 
-{* ../../docs_src/custom_response/tutorial007_py310.py hl[2,14] *}
+{* ../../docs_src/custom_response/tutorial007_py310.py hl[4,17] *}
+
+/// note
+
+An `async` task can only be cancelled when it reaches an `await`. If there is no `await`, the generator can not be cancelled properly and may keep running even after cancellation is requested.
+
+Since this small example does not need any `await` statements, we add an `await asyncio.sleep(0)` to give the event loop a chance to handle cancellation.
+
+///
 
 #### Using `StreamingResponse` with file-like objects { #using-streamingresponse-with-file-like-objects }
 

docs_src/custom_response/tutorial007_py310.py

@@ -1,3 +1,5 @@
+import asyncio
+
 from fastapi import FastAPI
 from fastapi.responses import StreamingResponse
 
@@ -7,6 +9,7 @@ app = FastAPI()
 async def fake_video_streamer():
     for i in range(10):
         yield b"some fake video bytes"
+        await asyncio.sleep(0)
 
 
 @app.get("/")