Enhance Readme Documentation to Clarify the Importance of Celery Service (#8558)

api/README.md

 
 8. Start Dify [web](../web) service.
 9. Setup your application by visiting `http://localhost:3000`...
-10. If you need to debug local async processing, please start the worker service.
+10. If you need to handle and debug the async tasks (e.g. dataset importing and documents indexing), please start the worker service.
 
    ```bash
    poetry run python -m celery -A app.celery worker -P gevent -c 1 --loglevel INFO -Q dataset,generation,mail,ops_trace,app_deletion
    ```
 
-   The started celery app handles the async tasks, e.g. dataset importing and documents indexing.
-
 ## Testing
 
 1. Install dependencies for both the backend and the test environment