documentation for creating observable streams

I've got a functions:

```python
def get_producer():
tool = Tool()

async def start(listener):
def on_event(event):
listener(event)
tool.on_scan_result = on_event
tool.start_scan()
await tool.transport.drain()

async def stop():
tool.stop_scanning()
await tool.transport.drain()

return { "start": start, "stop": stop } # my old api.
return AsyncObservable( # what goes here? )
```

And I'd like to be able to turn it into an observable stream, that automatically starts/stops scanning when the obserables are added/removed but I'm not sure how.

Start with the get_producer example and the AsyncObservable, Tool, start, and stop entry points named in the issue. Determine which existing documentation explains observable lifecycle management, then document how the example should represent subscription and disposal behavior; no documentation file or test is named, so the repository’s current observable examples are the first place to look.

Written by the indexing model from the issue text.