📝 document file sink policy mutators

docs/api/file-sink-set-append-mode.md

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+---
+name: file-sink-set-append-mode
+group: api
+category: sink
+update-time: 20260613
+description: Update the append-mode policy used by a FileSink for later reopen behavior.
+key-word:
+    - file
+    - sink
+    - append
+    - public
+---
+
+## File-sink-set-append-mode
+
+Update the append-mode policy used by a `FileSink`. This helper changes future reopen behavior on the concrete sink without forcing an immediate reopen.
+
+### Interface
+
+```moonbit
+pub fn FileSink::set_append_mode(self : FileSink, append : Bool) -> Unit {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose append-mode policy should change.
+- `append : Bool` - New append-mode policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This method updates stored policy only.
+- It does not force an immediate reopen.
+- The new value affects later reopen behavior.
+- The sink path and current rotation policy are left unchanged.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Change Future Reopen Behavior
+
+When direct recovery code should later prefer append mode explicitly:
+```moonbit
+sink.set_append_mode(true)
+```
+
+In this example, later reopen operations use the updated append policy.
+
+#### When Want To Separate Policy Mutation From Reopen Timing
+
+When code should update policy now and reopen later:
+```moonbit
+sink.set_append_mode(false)
+ignore(sink.reopen_with_current_policy())
+```
+
+In this example, policy mutation and reopen timing remain separate decisions.
+
+### Error Case
+
+e.g.:
+- If callers need an immediate reopen as part of the same operation, one of the reopen helpers should be used afterward.
+
+- This method does not itself prove that the sink is currently available or writable.
+
+### Notes
+
+1. This helper changes stored policy, not live file-handle state by itself.
+
+2. It is useful when direct file-sink recovery logic wants to stage reopen behavior in advance.