📝 document sync config model aliases

docs/api/sink-config-type.md

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+---
+name: sink-config-type
+group: api
+category: config
+update-time: 20260613
+description: Public sink config alias used for serializable built-in sink selection and settings.
+key-word:
+    - sink
+    - config
+    - alias
+    - public
+---
+
+## Sink-config-type
+
+`SinkConfig` is the public serializable config type used to describe the built-in sink shape and its related settings. It is a direct alias to the sink config model used by config parsing, logger building, and sink config serializers.
+
+### Interface
+
+```moonbit
+pub type SinkConfig = @utils.SinkConfig
+```
+
+#### output
+
+- `SinkConfig` - Public sink config object containing `kind`, `path`, `append`, `auto_flush`, `rotation`, and `text_formatter`.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a type alias, not a runtime sink instance.
+- The current fields are `kind : SinkKind`, `path : String`, `append : Bool`, `auto_flush : Bool`, `rotation : FileRotation?`, and `text_formatter : TextFormatterConfig`.
+- `SinkConfig::new(...)` constructs this type as the main code-side entry point.
+- `sink_config_to_json(...)`, `stringify_sink_config(...)`, and `build_logger(...)` all consume the same public config shape.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Typed Built-in Sink Description
+
+When sink selection and file/text settings should remain config data before logger construction:
+```moonbit
+let sink : SinkConfig = SinkConfig::new(kind=SinkKind::TextConsole)
+```
+
+In this example, the sink choice stays as a typed config object instead of becoming a runtime sink immediately.
+
+#### When Need To Inspect Or Export Sink Settings
+
+When a sink definition should be serialized or reviewed before build time:
+```moonbit
+let sink = SinkConfig::new(kind=SinkKind::File, path="app.log")
+println(stringify_sink_config(sink, pretty=true))
+```
+
+In this example, the same public config type supports both inspection and later runtime assembly.
+
+### Error Case
+
+e.g.:
+- `SinkConfig` itself does not have a runtime failure mode.
+
+- File-oriented fields can still be present on a non-file sink config, but those values may remain unused by the runtime builder.
+
+### Notes
+
+1. Use `SinkConfig::new(...)` when you need a value of this type in code.
+
+2. Use concrete sink constructors when you need a runtime sink directly instead of serializable config data.