📝 Add async state and config export API docs

docs/api/stringify-async-logger-config.md

@@ -0,0 +1,101 @@
+---
+name: stringify-async-logger-config
+group: api
+category: async
+update-time: 20260512
+description: Serialize AsyncLoggerConfig into compact or pretty JSON text for export and diagnostics.
+key-word:
+    - async
+    - config
+    - stringify
+    - public
+---
+
+## Stringify-async-logger-config
+
+Serialize `AsyncLoggerConfig` into JSON text. This helper is the most direct output path when async runtime policy should be logged, printed, or copied as config text.
+
+### Interface
+
+```moonbit
+pub fn stringify_async_logger_config(config : AsyncLoggerConfig, pretty~ : Bool = false) -> String {}
+```
+
+#### input
+
+- `config : AsyncLoggerConfig` - Async config to serialize.
+- `pretty : Bool` - Whether JSON should be pretty-printed.
+
+#### output
+
+- `String` - Serialized JSON text for the async config.
+
+---
+
+e.g.:
+```moonbit
+pub fn stringify_async_logger_config(config : AsyncLoggerConfig, pretty~ : Bool = false) -> String {}
+```
+
+#### input
+
+- `config : AsyncLoggerConfig` - Config to stringify.
+- `pretty : Bool` - Pretty-print flag.
+
+#### output
+
+- `String` - JSON text.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- `pretty=false` returns compact JSON suitable for transport and snapshots.
+- `pretty=true` returns indented JSON for humans.
+- This helper is built on top of `async_logger_config_to_json(...)`.
+- The exported text follows the supported async config schema rather than internal queue implementation details.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Human-readable Async Config
+
+When async policy should be printed during startup or testing:
+```moonbit
+println(stringify_async_logger_config(AsyncLoggerConfig::new(max_pending=32), pretty=true))
+```
+
+In this example, the config is emitted in readable JSON.
+
+#### When Need Compact Generated Config Text
+
+When async settings should stay small:
+```moonbit
+let text = stringify_async_logger_config(
+  AsyncLoggerConfig::new(flush=AsyncFlushPolicy::Shutdown),
+)
+```
+
+In this example, compact JSON is returned without extra formatting.
+
+### Error Case
+
+e.g.:
+- If callers need a `JsonValue` for composition, they should use `async_logger_config_to_json(...)` instead.
+
+- If invalid constructor inputs were normalized earlier, the resulting text contains the normalized config values.
+
+### Notes
+
+Notes are here.
+
+1. This API is convenient for config snapshots, examples, and tests.
+
+2. Use `pretty=true` when readability matters.
+
+3. Compact mode is a better default for machine-oriented output.
+
+4. This helper exports async config data, not runtime logger state.