📝 Add async logger write API docs

docs/api/async-logger-warn.md

@@ -0,0 +1,83 @@
+---
+name: async-logger-warn
+group: api
+category: async
+update-time: 20260512
+description: Enqueue a warning-level record through the async logger using the built-in severity shortcut.
+key-word:
+    - async
+    - logger
+    - warn
+    - public
+---
+
+## Async-logger-warn
+
+Enqueue a warning-level record through the async logger. This is the convenience wrapper for `log(Level::Warn, ...)`.
+
+### Interface
+
+```moonbit
+pub async fn[S] AsyncLogger::warn(
+  self : AsyncLogger[S],
+  message : String,
+  fields~ : Array[@bitlogger.Field] = [],
+) -> Unit {}
+```
+
+#### input
+
+- `self : AsyncLogger[S]` - Async logger that should receive the warning record.
+- `message : String` - Warning message text.
+- `fields : Array[Field]` - Optional structured fields added to the record.
+
+#### output
+
+- `Unit` - No return value. The record is handled according to logger state and policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper delegates to `log(Level::Warn, ...)`.
+- The record is still subject to min-level gating, patching, filtering, and overflow policy.
+- Warning records are useful for degraded but non-fatal runtime conditions.
+- Use this helper when a named warning call is clearer than a raw `log(...)` call.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Async Degradation Signals
+
+When the system should report a non-fatal problem:
+```moonbit
+await logger.warn("retry budget running low")
+```
+
+In this example, the event is surfaced at warning severity without using the generic `log(...)` form.
+
+#### When Attach Structured Warning Detail
+
+When a warning event should include context:
+```moonbit
+await logger.warn(
+  "queue near capacity",
+  fields=[@bitlogger.field("pending", logger.pending_count().to_string())],
+)
+```
+
+In this example, the warning carries structured operational detail.
+
+### Error Case
+
+e.g.:
+- If the logger minimum level is above `Warn`, the record is skipped before enqueue.
+
+- If the logger is closed or overflow policy prevents acceptance, the write may not become a normal queued record.
+
+### Notes
+
+1. Use this helper for notable but non-fatal async runtime conditions.
+
+2. Pair warnings with structured fields when operators need quick context.