📝 document queued sink methods

docs/api/queued-sink-flush.md

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+---
+name: queued-sink-flush
+group: api
+category: sink
+update-time: 20260613
+description: Flush a QueuedSink by draining its queued records into the wrapped sink.
+key-word:
+    - sink
+    - queue
+    - flush
+    - public
+---
+
+## Queued-sink-flush
+
+Flush a `QueuedSink[S]` by draining its queued records into the wrapped sink. This is the direct sink-level shorthand for draining the whole queue.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] QueuedSink::flush(self : QueuedSink[S]) -> Int {
+```
+
+#### input
+
+- `self : QueuedSink[S]` - Queued sink whose pending records should be fully drained to the wrapped sink.
+
+#### output
+
+- `Int` - Number of drained records.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This method delegates directly to `self.drain()`.
+- It drains as many queued records as are currently pending.
+- The return value is the number of records forwarded to the wrapped sink.
+- This helper is the sink-level shorthand for a full manual queue drain.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Full Queue Flush Semantics
+
+When a queued sink should forward all pending records:
+```moonbit
+let flushed = sink.flush()
+```
+
+In this example, callers can observe how many queued records were forwarded.
+
+#### When Need A Simpler Alternative To Explicit Drain Limits
+
+When bounded draining is unnecessary and the whole queue should be released:
+```moonbit
+ignore(sink.flush())
+```
+
+In this example, the queue is drained completely without specifying `max_items`.
+
+### Error Case
+
+e.g.:
+- If the queue is already empty, the method returns `0`.
+
+- If callers need bounded progress instead of a full drain, `drain(max_items=...)` is the better API.
+
+### Notes
+
+1. Use this helper when code owns a `QueuedSink` directly and wants a full manual queue release.
+
+2. This helper returns a count, unlike `BufferedSink::flush()`.