📝 Add logger write API docs

docs/api/logger-warn.md

@@ -0,0 +1,77 @@
+---
+name: logger-warn
+group: api
+category: logging
+update-time: 20260512
+description: Emit a warn-level record through the logger using the warning severity shortcut.
+key-word:
+    - logger
+    - warn
+    - sync
+    - public
+---
+
+## Logger-warn
+
+Emit a warn-level record through the synchronous logger. This is the convenience wrapper for `log(Level::Warn, ...)`.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] Logger::warn(self : Logger[S], message : String, fields~ : Array[Field] = []) -> Unit {}
+```
+
+#### input
+
+- `self : Logger[S]` - Logger that should emit the warning record.
+- `message : String` - Warning message text.
+- `fields : Array[Field]` - Optional structured fields attached to the record.
+
+#### output
+
+- `Unit` - No return value. The record is handled according to the logger threshold and sink pipeline.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper delegates to `log(Level::Warn, ...)`.
+- Warning records are useful for abnormal but non-fatal conditions.
+- Per-call target override is not exposed here; use `log(...)` when that is required.
+- All logger wrappers still participate normally in the write path.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Signal A Recoverable Problem
+
+When an operation degraded but still continued:
+```moonbit
+logger.warn("cache miss ratio increased")
+```
+
+In this example, the event is elevated above normal information without being treated as a hard failure.
+
+#### When Attach Structured Warning Context
+
+When a warning should include machine-readable detail:
+```moonbit
+logger.warn("retry scheduled", fields=[field("attempt", "3")])
+```
+
+In this example, the warning remains easy to filter and inspect later.
+
+### Error Case
+
+e.g.:
+- If the logger minimum level is above `Warn`, the call returns without writing a record.
+
+- If a target override is needed at this call site, use `log(...)` instead of this shortcut.
+
+### Notes
+
+1. Use this helper for degraded or suspicious states that do not stop execution.
+
+2. Warning logs are often a practical signal threshold for alerting or separate routing.
+