📝 Add await in StreamingResponse code example to allow cancellation (#14681)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

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

@@ -167,7 +167,17 @@ 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[3,16] *}
+
+/// note | Technical Details
+
+An `async` task can only be cancelled when it reaches an `await`. If there is no `await`, the generator (function with `yield`) 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 anyio.sleep(0)` to give the event loop a chance to handle cancellation.
+
+This would be even more important with large or infinite streams.
+
+///
 
 #### Using `StreamingResponse` with file-like objects { #using-streamingresponse-with-file-like-objects }
 

docs_src/custom_response/tutorial007_py310.py

@@ -1,3 +1,4 @@
+import anyio
 from fastapi import FastAPI
 from fastapi.responses import StreamingResponse
 
@@ -7,6 +8,7 @@ app = FastAPI()
 async def fake_video_streamer():
     for i in range(10):
         yield b"some fake video bytes"
+        await anyio.sleep(0)
 
 
 @app.get("/")