📝 document file sink reopen methods

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

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+---
+name: file-sink-reopen-append
+group: api
+category: sink
+update-time: 20260613
+description: Reopen a FileSink in append mode.
+key-word:
+    - file
+    - sink
+    - reopen
+    - public
+---
+
+## File-sink-reopen-append
+
+Reopen a `FileSink` in append mode. This helper is the explicit append-oriented recovery shortcut on the direct sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::reopen_append(self : FileSink) -> Bool {
+```
+
+#### input
+
+- `self : FileSink` - File sink that should be reopened in append mode.
+
+#### output
+
+- `Bool` - Whether reopen succeeded.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper is a specialized shortcut over `reopen(append=Some(true))`.
+- Reopen behavior is fixed to append mode.
+- The stored append policy is updated to `true` as part of the reopen path.
+- On failure, the sink remains unavailable and `open_failures` is incremented.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Append-preserving Recovery
+
+When file logging should continue appending after a reopen:
+```moonbit
+ignore(sink.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.reopen_append()
+```
+
+In this example, the call site states append intent directly.
+
+### Error Case
+
+e.g.:
+- If callers need truncate behavior instead, `reopen_truncate()` is the correct API.
+
+- If reopen fails, the helper returns `false` and the sink stays unavailable.
+
+### Notes
+
+1. Use this helper for explicit append-mode reopen flows.
+
+2. It is especially useful after transient file availability issues.