📝 Update More API Document

docs/api/queued-sink.md

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+---
+name: queued-sink
+group: api
+category: sink
+update-time: 20260520
+description: Create a sink that buffers records in an explicit queue with overflow policy control.
+key-word:
+    - sink
+    - queue
+    - overflow
+    - public
+---
+
+## Queued-sink
+
+Create a sink that stores records in an explicit synchronous queue before forwarding them to another sink. This is the low-level sink constructor behind queue-style sync buffering behavior.
+
+### Interface
+
+```moonbit
+pub fn[S] queued_sink(
+  sink : S,
+  max_pending~ : Int = 0,
+  overflow~ : QueueOverflowPolicy = QueueOverflowPolicy::DropNewest,
+) -> QueuedSink[S] {
+```
+
+#### input
+
+- `sink : S` - Wrapped sink receiving drained records.
+- `max_pending : Int` - Queue capacity before overflow policy applies. `0` means unbounded.
+- `overflow : QueueOverflowPolicy` - Overflow behavior such as `DropNewest` or `DropOldest`.
+
+#### output
+
+- `QueuedSink[S]` - Explicit queue sink with `pending_count()`, `dropped_count()`, `drain()`, and `flush()` helpers.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Records are enqueued first and only reach the wrapped sink when drained or flushed.
+- `max_pending=0` means the queue is not bounded by capacity.
+- Overflow behavior controls whether the incoming record is dropped or the oldest queued record is replaced.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Low-level Explicit Queue Composition
+
+When a sync logger should own a queue sink directly:
+```moonbit
+let sink = queued_sink(console_sink(), max_pending=2, overflow=QueueOverflowPolicy::DropOldest)
+let logger = Logger::new(sink, target="queue")
+```
+
+In this example, records stay pending until the queue is drained or flushed.
+
+### Error Case
+
+e.g.:
+- If the queue is bounded too tightly, overflow may drop records during bursts.
+
+- If callers never drain or flush, queued records remain pending.
+
+### Notes
+
+1. This helper is the sink-level alternative to `Logger::with_queue(...)` and config-side `with_queue(...)`.
+
+2. Use the higher-level APIs when you do not need to manipulate the queue sink directly.