📝 clarify built sync target behavior

docs/api/build-logger.md

@@ -36,6 +36,7 @@ Detailed rules explaining key parameters and behaviors
 - `build_logger(...)` first constructs a base `RuntimeSink` from `config.sink`, then applies `config.queue` when present, and finally builds `Logger::new(...)` with `config.min_level`, `config.target`, and `config.timestamp`.
 - The returned logger still supports normal logging methods because `ConfiguredLogger` is `Logger[RuntimeSink]`.
 - The returned logger also keeps inherited logger target behavior such as `with_target(...)`, `child(...)`, and per-call `target=` overrides on `log(...)`.
+- That means `log(..., target=...)` can override the target for one write, while severity helpers such as `info(...)`, `warn(...)`, and `error(...)` continue to use the stored logger target unless a derived logger was created first with `with_target(...)` or `child(...)`.
 - Queue metrics and file controls remain available through forwarding helpers on the configured logger.
 - `build_application_logger(...)` only re-exports this same configured runtime logger result under the `ApplicationLogger` alias, while `build_library_logger(...)` wraps the same result in `LibraryLogger[RuntimeSink]`.
 - This API is deterministic and data-driven, making it suitable for bootstrapping from parsed config.
@@ -61,6 +62,18 @@ In this example, no JSON parsing is required because config objects were built d
 
 And the runtime logger is ready immediately, with the same ordinary logger target semantics as any other `Logger` value.
 
+#### When Need A Per-call Target Override After Typed Config Build
+
+When typed config should still build a logger that supports a one-write target override:
+```moonbit
+let logger = build_logger(config)
+logger.log(Level::Error, "boom", target="svc.audit")
+```
+
+In this example, the emitted record uses `svc.audit` for that write.
+
+And later `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 Config-built Queue Or File Runtime Helpers
 
 When the sink shape comes from config but runtime controls still matter: