📝 document file sink policy mutators

docs/api/file-sink-set-auto-flush.md

@@ -0,0 +1,71 @@
+---
+name: file-sink-set-auto-flush
+group: api
+category: sink
+update-time: 20260613
+description: Update the auto-flush policy used by a FileSink.
+key-word:
+    - file
+    - sink
+    - flush
+    - public
+---
+
+## File-sink-set-auto-flush
+
+Update the auto-flush policy used by a `FileSink`. This helper changes direct runtime durability behavior without rebuilding the sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::set_auto_flush(self : FileSink, enabled : Bool) -> Unit {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose auto-flush policy should change.
+- `enabled : Bool` - New auto-flush setting.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This method updates runtime policy only.
+- It does not itself flush pending data.
+- The new value affects whether later writes attempt a flush after each rendered record.
+- Existing path, append policy, and rotation policy are left unchanged.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Direct Durability Tuning
+
+When a concrete file sink should start flushing each write automatically:
+```moonbit
+sink.set_auto_flush(true)
+```
+
+In this example, runtime policy is updated without rebuilding the sink.
+
+#### When Need To Relax Flush Pressure
+
+When a file sink should stop auto-flushing for throughput reasons:
+```moonbit
+sink.set_auto_flush(false)
+```
+
+In this example, the call site updates policy explicitly on the direct file sink.
+
+### Error Case
+
+e.g.:
+- If callers need an immediate flush action, `flush()` is the operational API.
+
+- This helper does not report whether previous writes or flushes succeeded.
+
+### Notes
+
+1. Use this helper for direct runtime durability tuning.
+
+2. Pair it with `auto_flush_enabled()` to verify the active policy.