📝 Add configured logger runtime control API docs

docs/api/configured-logger-drain.md

@@ -0,0 +1,75 @@
+---
+name: configured-logger-drain
+group: api
+category: runtime
+update-time: 20260512
+description: Drain queued work from a configured runtime logger with optional item limits.
+key-word:
+    - logger
+    - runtime
+    - queue
+    - public
+---
+
+## Configured-logger-drain
+
+Drain queued work from a `ConfiguredLogger`. This helper is useful when config-driven queue wrapping should be advanced in a controlled, bounded way.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::drain(self : ConfiguredLogger, max_items~ : Int = -1) -> Int {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose queued work should be drained.
+- `max_items : Int` - Optional upper bound on how many queued items to drain. Negative values mean no explicit bound.
+
+#### output
+
+- `Int` - Number of drained items.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Queue-wrapped sinks may drain up to `max_items` records.
+- For plain file sinks, the runtime falls back to file flush behavior instead of queue draining.
+- For sinks without queue semantics, the result is typically `0`.
+- This helper delegates to `RuntimeSink::drain(...)` through the configured logger wrapper.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Bounded Queue Progress
+
+When queued output should be advanced in chunks:
+```moonbit
+let drained = logger.drain(max_items=16)
+```
+
+In this example, callers limit how much queued work is processed in one step.
+
+#### When Need Full Manual Drain
+
+When the configured queue should be emptied explicitly:
+```moonbit
+ignore(logger.drain())
+```
+
+In this example, the configured runtime logger drains without imposing an item cap.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not queue-backed, draining may return `0` or follow fallback flush 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 observable.
+
+2. Use `pending_count()` to inspect remaining backlog after the drain call.