📝 document runtime sink control methods

docs/api/runtime-sink-drain.md

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+---
+name: runtime-sink-drain
+group: api
+category: runtime
+update-time: 20260613
+description: Drain queued work from a RuntimeSink with an optional item limit.
+key-word:
+    - runtime
+    - sink
+    - queue
+    - public
+---
+
+## Runtime-sink-drain
+
+Drain queued work from a `RuntimeSink`. This helper is useful when code owns a runtime sink directly and needs controlled queue progress.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::drain(self : RuntimeSink, max_items~ : Int = -1) -> Int {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose queued work should be drained.
+- `max_items : Int` - Optional upper bound on drained queued items. Negative values mean no explicit bound.
+
+#### output
+
+- `Int` - Number of drained items, or the fallback flush count for plain file sinks.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Queue-wrapped runtime sinks forward to the wrapped queue sink's `drain(...)` behavior.
+- Plain file runtime sinks fall back to `FileSink::flush()` behavior and return `1` or `0`.
+- Plain console-style runtime sinks return `0` because they do not own a drainable queue.
+- This method is the direct sink-level API used by `ConfiguredLogger::drain(...)`.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Bounded Queue Progress
+
+When direct queue-backed runtime work should be advanced in chunks:
+```moonbit
+let drained = sink.drain(max_items=16)
+```
+
+In this example, callers cap how much queued work is processed in one step.
+
+#### When Need Full Manual Drain
+
+When a runtime queue should be emptied without an explicit item cap:
+```moonbit
+ignore(sink.drain())
+```
+
+In this example, the runtime sink drains as much queued work as its concrete variant allows.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not queue-backed, the result may be `0` or file-flush fallback behavior.
+
+- If callers only need generic flush semantics, `flush()` may be the simpler API.
+
+### Notes
+
+1. Prefer this helper when queue progress should be bounded or directly observed.
+
+2. `ConfiguredLogger::drain(...)` is the higher-level wrapper for config-built loggers.