📝 document file sink policy state methods

docs/api/file-sink-set-rotation.md

@@ -0,0 +1,71 @@
+---
+name: file-sink-set-rotation
+group: api
+category: sink
+update-time: 20260613
+description: Update the rotation configuration used by a FileSink.
+key-word:
+    - file
+    - sink
+    - rotation
+    - public
+---
+
+## File-sink-set-rotation
+
+Update the rotation configuration used by a `FileSink`. This helper changes direct runtime file rotation behavior without rebuilding or reopening the sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::set_rotation(self : FileSink, rotation : FileRotation?) -> Unit {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose rotation policy should change.
+- `rotation : FileRotation?` - New rotation config, or `None` to disable rotation.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This method updates runtime rotation policy only.
+- Passing `None` disables rotation.
+- It does not reopen the sink or rotate immediately by itself.
+- Append and auto-flush settings are left unchanged.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Rotation Tuning
+
+When a concrete file sink should enable or change rotation dynamically:
+```moonbit
+sink.set_rotation(Some(file_rotation(1024, max_backups=3)))
+```
+
+In this example, direct runtime rotation behavior is updated without rebuilding the sink.
+
+#### When Need To Disable Rotation
+
+When the file sink should stop rotating:
+```moonbit
+sink.set_rotation(None)
+```
+
+In this example, the sink has its rotation policy cleared explicitly.
+
+### Error Case
+
+e.g.:
+- If callers only want to remove rotation, `clear_rotation()` is the more direct API.
+
+- This helper does not report past rotation failures; use `rotation_failures()` or `state()` for diagnostics.
+
+### Notes
+
+1. Use this helper when setting a full runtime rotation config.
+
+2. It is useful for operational tuning on a concrete file sink without rebuilding it.