📝 document runtime sink policy setter

docs/api/index.md

@@ -371,6 +371,7 @@ BitLogger API navigation.
 - [runtime-sink-file-set-append-mode.md](./runtime-sink-file-set-append-mode.md)
 - [runtime-sink-file-auto-flush.md](./runtime-sink-file-auto-flush.md)
 - [runtime-sink-file-set-auto-flush.md](./runtime-sink-file-set-auto-flush.md)
+- [runtime-sink-file-set-policy.md](./runtime-sink-file-set-policy.md)
 - [runtime-sink-file-rotation-enabled.md](./runtime-sink-file-rotation-enabled.md)
 - [runtime-sink-file-rotation-config.md](./runtime-sink-file-rotation-config.md)
 - [runtime-sink-file-set-rotation.md](./runtime-sink-file-set-rotation.md)

docs/api/runtime-sink-file-set-policy.md

@@ -0,0 +1,79 @@
+---
+name: runtime-sink-file-set-policy
+group: api
+category: runtime
+update-time: 20260613
+description: Apply a bundled runtime file policy update to a file-backed RuntimeSink.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-set-policy
+
+Apply a bundled runtime file policy update to a `RuntimeSink`. This helper updates append mode, auto-flush, and rotation together through one runtime policy object on direct sink values.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_set_policy(self : RuntimeSink, policy : FileSinkPolicy) -> Bool {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose file policy should change.
+- `policy : FileSinkPolicy` - Bundled runtime file policy to apply.
+
+#### output
+
+- `Bool` - Whether the policy update was applied.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants update append, auto-flush, and rotation together on the wrapped `FileSink` and return `true`.
+- `QueuedFile` runtime variants forward the policy update to the wrapped inner `FileSink` and return `true`.
+- Non-file runtime variants return `false`.
+- This helper is broader than the single-setting setters because it updates the whole file policy in one call.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Bundled Runtime Policy Changes
+
+When append, flush, and rotation should change together:
+```moonbit
+ignore(sink.file_set_policy(FileSinkPolicy::new(
+  append=true,
+  auto_flush=false,
+  rotation=Some(file_rotation(2048, max_backups=2)),
+)))
+```
+
+In this example, runtime file behavior is updated as one policy change.
+
+#### When Apply A Policy Snapshot
+
+When a previously captured or computed policy should be restored:
+```moonbit
+let ok = sink.file_set_policy(policy)
+```
+
+In this example, callers can reapply a whole policy object without splitting it into separate setter calls.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the method returns `false`.
+
+- If callers only need to change one setting, a narrower setter such as `file_set_auto_flush(...)` or `file_set_rotation(...)` may be clearer.
+
+### Notes
+
+1. Use this helper when the runtime policy should be treated as one cohesive object.
+
+2. It pairs naturally with `file_policy()` and `file_default_policy()`.