📝 document runtime sink reopen methods

docs/api/runtime-sink-file-reopen-append.md

@@ -0,0 +1,74 @@
+---
+name: runtime-sink-file-reopen-append
+group: api
+category: runtime
+update-time: 20260613
+description: Reopen the file sink behind a RuntimeSink in append mode.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-reopen-append
+
+Reopen the file sink behind a `RuntimeSink` in append mode. This helper is the explicit append-oriented direct recovery shortcut.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_reopen_append(self : RuntimeSink) -> Bool {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose file sink should be reopened in append mode.
+
+#### output
+
+- `Bool` - Whether reopen succeeded.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants reopen in append mode.
+- `QueuedFile` runtime variants forward reopen behavior to the wrapped inner file sink.
+- This helper is a specialized shortcut for a common reopen mode.
+- Non-file runtime variants return `false`.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Append-preserving Direct Recovery
+
+When file logging should continue appending after a reopen:
+```moonbit
+ignore(sink.file_reopen_append())
+```
+
+In this example, reopen behavior is fixed to append mode.
+
+#### When Want An Explicit Append Shortcut
+
+When code should avoid manually setting append overrides:
+```moonbit
+let ok = sink.file_reopen_append()
+```
+
+In this example, the call site states append intent directly.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the method returns `false`.
+
+- If callers need truncate behavior instead, `file_reopen_truncate()` is the correct API.
+
+### Notes
+
+1. Use this helper for explicit append-mode reopen flows on direct `RuntimeSink` values.
+
+2. It is especially useful after transient file availability issues.