📝 Add configured logger file control API docs

docs/api/configured-logger-file-set-append-mode.md

@@ -0,0 +1,76 @@
+---
+name: configured-logger-file-set-append-mode
+group: api
+category: runtime
+update-time: 20260512
+description: Update the append-mode policy used by the configured runtime file sink for later reopen behavior.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-set-append-mode
+
+Update the append-mode policy used by a `ConfiguredLogger` file sink. This helper changes future reopen behavior without forcing an immediate reopen.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_set_append_mode(self : ConfiguredLogger, append : Bool) -> Bool {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose append-mode policy should change.
+- `append : Bool` - New append-mode policy.
+
+#### output
+
+- `Bool` - Whether the policy update was applied.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks update their stored append policy.
+- Queued file sinks forward the policy update to the wrapped file sink.
+- This helper updates policy only; it does not force immediate reopen.
+- Non-file sinks return `false`.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Change Future Reopen Behavior
+
+When runtime recovery should later prefer append mode explicitly:
+```moonbit
+ignore(logger.file_set_append_mode(true))
+```
+
+In this example, later reopen operations will use the updated append policy.
+
+#### When Want To Separate Policy Mutation From Reopen Timing
+
+When code should update policy now and reopen later:
+```moonbit
+ignore(logger.file_set_append_mode(false))
+ignore(logger.file_reopen_with_current_policy())
+```
+
+In this example, policy mutation and reopen timing remain separate decisions.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the method returns `false`.
+
+- If callers need an immediate reopen as part of the same operation, one of the reopen helpers should be used afterward.
+
+### Notes
+
+1. This helper changes stored policy, not live file-handle state by itself.
+
+2. It is useful when recovery logic wants to stage reopen behavior in advance.