📝 clarify root logger target behavior

docs/api/logger.md

@@ -37,6 +37,7 @@ Detailed rules explaining key parameters and behaviors
 - This is a public root struct, not a type alias.
 - The current fields are `min_level : Level`, `sink : S`, `target : String`, and `timestamp : Bool`.
 - The sink type parameter is preserved across composition, which is why helpers such as `with_context_fields(...)`, `with_filter(...)`, `with_patch(...)`, and `with_queue(...)` can return more specific logger shapes.
+- The root logger also preserves the core target contract used across the sync API surface: `log(..., target=...)` can override the target for one call, while fixed-level helpers such as `trace(...)`, `debug(...)`, `info(...)`, `warn(...)`, and `error(...)` continue using the stored logger target unless code derives another logger first with `with_target(...)` or `child(...)`.
 - `Logger::new(...)` constructs this type as the main synchronous entry point.
 - This root type is also what sits underneath both facade families: `ApplicationLogger` is a direct alias over the configured `Logger[RuntimeSink]` line, while `LibraryLogger[S]` is a narrowing wrapper around a `Logger[S]` value.
 
@@ -53,6 +54,17 @@ let logger : Logger[ConsoleSink] = Logger::new(console_sink(), target="app")
 
 In this example, the root logger keeps the concrete console sink type visible for later typed composition.
 
+#### When Need A One-call Target Override On The Root Logger
+
+When sync code should keep one logger value but emit a single record under a different target:
+```moonbit
+logger.log(Level::Error, "boom", target="app.audit")
+```
+
+In this example, the emitted record uses `app.audit` only for that one call.
+
+And later `trace(...)`, `debug(...)`, `info(...)`, `warn(...)`, or `error(...)` calls still use the logger's stored target unless code derives another logger first with `with_target(...)` or `child(...)`.
+
 #### When Need To Build A Composed Logging Pipeline
 
 When code should start from one root logger and then derive more specific wrapped forms: