📝 clarify async backlog docs

docs/api/async-logger-wait-idle.md

@@ -36,6 +36,7 @@ Detailed rules explaining key parameters and behaviors
 - The helper keeps yielding while `pending_count() > 0`.
 - If `has_failed()` becomes `true`, waiting stops early instead of looping forever.
 - This API does not close the logger or stop the worker.
+- A return from `wait_idle()` therefore means either backlog reached `0` or failure interrupted normal drain progress.
 - It is narrower than `shutdown()` and is useful when the logger should continue to be used later.
 
 ### How to Use
@@ -68,8 +69,12 @@ e.g.:
 
 - If the worker was never started, pending records may not drain and callers should not expect idle progress automatically.
 
+- If callers need backlog cleanup after a failure-short-circuit, they still need a later `close(clear=true)` or `shutdown(...)` path.
+
 ### Notes
 
 1. Use this helper when you want a drain barrier without closing the logger.
 
 2. Prefer `shutdown()` when lifecycle completion matters more than continued reuse.
+
+3. Pair it with `pending_count()` or `state()` when you need to distinguish true idleness from an early stop caused by worker failure.