📝 Add global logger API docs

docs/api/set-default-target.md

@@ -0,0 +1,77 @@
+---
+name: set-default-target
+group: api
+category: global
+update-time: 20260512
+description: Update the default target used by the shared global logger helpers.
+key-word:
+    - global
+    - target
+    - default
+    - public
+---
+
+## Set-default-target
+
+Update the default target used by the shared default logger returned by `default_logger()` and by the global write helpers such as `info(...)` and `error(...)`.
+
+### Interface
+
+```moonbit
+pub fn set_default_target(target : String) -> Unit {}
+```
+
+#### input
+
+- `target : String` - New default target string for the shared global logger configuration.
+
+#### output
+
+- `Unit` - No return value. The stored default target is updated for later global calls.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The function updates the internal `Ref[String]` used by `default_logger()`.
+- It does not mutate any custom `Logger` values created earlier.
+- Later global writes inherit the new target unless a different explicit logger path is used.
+- This is useful when one application wants the convenience of global helpers while still labeling records consistently.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Label Global Application Logs
+
+When global helper calls should carry one stable target:
+```moonbit
+set_default_target("app")
+info("service started")
+```
+
+In this example, the emitted global record uses `app` as its target.
+
+#### When Re-scope A Temporary Execution Context
+
+When a script or tool should tag its own global output:
+```moonbit
+set_default_target("tool.migration")
+warn("running fallback path")
+```
+
+In this example, later global writes stay grouped under the migration target.
+
+### Error Case
+
+e.g.:
+- If `target` is empty, global writes remain valid and simply omit the target unless another logger path supplies one.
+
+- Previously created custom loggers are unaffected because they do not read the shared default target reference afterward.
+
+### Notes
+
+1. This API is best suited for small apps, scripts, or a single shared logging entry path.
+
+2. Prefer explicit child or per-component loggers when target structure needs to vary across subsystems.
+