📝 Add sync config export API docs

docs/api/stringify-sink-config.md

@@ -0,0 +1,99 @@
+---
+name: stringify-sink-config
+group: api
+category: config
+update-time: 20260512
+description: Serialize SinkConfig into compact or pretty JSON text.
+key-word:
+    - sink
+    - config
+    - stringify
+    - public
+---
+
+## Stringify-sink-config
+
+Serialize `SinkConfig` into JSON text. This helper is the direct text output path for sink configuration when examples, tooling, or diagnostics want the sink section by itself.
+
+### Interface
+
+```moonbit
+pub fn stringify_sink_config(config : SinkConfig, pretty~ : Bool = false) -> String {}
+```
+
+#### input
+
+- `config : SinkConfig` - Sink config to serialize.
+- `pretty : Bool` - Whether output should be pretty-printed.
+
+#### output
+
+- `String` - Serialized sink config JSON.
+
+---
+
+e.g.:
+```moonbit
+pub fn stringify_sink_config(config : SinkConfig, pretty~ : Bool = false) -> String {}
+```
+
+#### input
+
+- `config : SinkConfig` - Sink config value.
+- `pretty : Bool` - Pretty-print option.
+
+#### output
+
+- `String` - JSON string.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- `pretty=false` gives compact JSON.
+- `pretty=true` gives indented output.
+- This helper builds on top of `sink_config_to_json(...)`.
+- It is useful when examples or generated docs want to show only sink-specific config.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Human-readable Sink Config
+
+When sink config should be printed for review:
+```moonbit
+println(stringify_sink_config(SinkConfig::new(kind=SinkKind::File, path="app.log"), pretty=true))
+```
+
+In this example, the sink section is easy to inspect visually.
+
+#### When Need Compact Embedded Output
+
+When sink config should be emitted in a compact form:
+```moonbit
+println(stringify_sink_config(config.sink))
+```
+
+In this example, the helper provides a ready-made machine-oriented representation.
+
+### Error Case
+
+e.g.:
+- If callers need a JSON value instead of text, use `sink_config_to_json(...)` instead.
+
+- If optional `rotation` is absent, the serialized sink config simply omits that field.
+
+### Notes
+
+Notes are here.
+
+1. This helper is convenient for documentation, examples, and generated config output.
+
+2. Use `pretty=true` when humans are the primary audience.
+
+3. The output describes the config schema, not live runtime sink state.
+
+4. This API pairs naturally with `SinkConfig::new(...)` and `sink_config_to_json(...)`.