📝 Add global logger API docs

docs/api/global-warn.md

@@ -0,0 +1,76 @@
+---
+name: global-warn
+group: api
+category: global
+update-time: 20260512
+description: Emit a warn-level record through the shared default logger shortcut.
+key-word:
+    - global
+    - warn
+    - default
+    - public
+---
+
+## Global-warn
+
+Emit a warn-level record through the shared default logger. This is the global convenience wrapper for `log(Level::Warn, ...)`.
+
+### Interface
+
+```moonbit
+pub fn warn(message : String, fields~ : Array[Field] = []) -> Unit {}
+```
+
+#### input
+
+- `message : String` - Warning message text.
+- `fields : Array[Field]` - Optional structured fields attached to the record.
+
+#### output
+
+- `Unit` - No return value. The record is handled through the shared default logger.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper delegates to `default_logger().warn(...)`.
+- The shared current threshold and target determine whether and how the record is emitted.
+- Warning logs are suited to degraded or suspicious states that do not necessarily stop execution.
+- This helper offers convenience for a single shared logging path.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Quick Global Warning
+
+When a recoverable issue should be surfaced without creating a logger variable:
+```moonbit
+warn("cache miss ratio increased")
+```
+
+In this example, the shared default logger emits a warning-severity record.
+
+#### When Attach Structured Warning Context
+
+When the warning should include machine-readable detail:
+```moonbit
+warn("retry scheduled", fields=[field("attempt", "3")])
+```
+
+In this example, the global helper still supports structured metadata.
+
+### Error Case
+
+e.g.:
+- If the shared minimum level is above `Warn`, the record is skipped.
+
+- If subsystem-specific routing is needed, a shared global warning path may be too limited.
+
+### Notes
+
+1. Global warnings are convenient for simple apps and scripts.
+
+2. Warning-level global events are often the first candidate for separate monitoring or routing later.
+