📝 document queued sink methods

docs/api/queued-sink-drain.md

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+---
+name: queued-sink-drain
+group: api
+category: sink
+update-time: 20260613
+description: Drain queued records from a QueuedSink with an optional item limit.
+key-word:
+    - sink
+    - queue
+    - drain
+    - public
+---
+
+## Queued-sink-drain
+
+Drain queued records from a `QueuedSink[S]`. This is the direct sink-level queue delivery API when code owns a queued sink and wants controlled progress into the wrapped sink.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] QueuedSink::drain(self : QueuedSink[S], max_items~ : Int = -1) -> Int {
+```
+
+#### input
+
+- `self : QueuedSink[S]` - Queued sink whose pending records should be drained to the wrapped sink.
+- `max_items : Int` - Optional upper bound on drained queued records. Negative values mean no explicit bound.
+
+#### output
+
+- `Int` - Number of drained records.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- `max_items == 0` returns `0` immediately.
+- Negative `max_items` means drain up to the current `pending_count()`.
+- The method repeatedly pops from the queue and writes each record to the wrapped sink in order until the limit is reached or the queue becomes empty.
+- The return value is the number of records actually drained.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Bounded Queue Progress
+
+When queued work should be advanced in chunks:
+```moonbit
+let drained = sink.drain(max_items=16)
+```
+
+In this example, callers cap how much queue backlog is forwarded in one step.
+
+#### When Need Full Manual Drain
+
+When a queued sink should be emptied explicitly:
+```moonbit
+ignore(sink.drain())
+```
+
+In this example, the queue is drained without an explicit item cap.
+
+### Error Case
+
+e.g.:
+- If the queue is already empty, the method returns `0`.
+
+- If callers only need the shorthand that drains the whole queue, `flush()` is the simpler API.
+
+### Notes
+
+1. Prefer this helper when queue progress should be bounded or directly observed.
+
+2. Pair it with `pending_count()` to inspect remaining backlog after a drain step.