Emphasize `asyncio` only with sleeps

docs/README.rst

@@ -347,6 +347,7 @@ Check out our experimental system for `guest-mode`_ controlled
         while True:
             # echo the msg back
             to_trio.send_nowait(await from_trio.get())
+            await asyncio.sleep(0)
 
 
     @tractor.context

examples/infected_asyncio_echo_server.py

@@ -25,6 +25,7 @@ async def aio_echo_server(
     while True:
         # echo the msg back
         to_trio.send_nowait(await from_trio.get())
+        await asyncio.sleep(0)
 
 
 @tractor.context