📝 Add configured logger file runtime API docs

docs/api/configured-logger-file-flush.md

@@ -0,0 +1,74 @@
+---
+name: configured-logger-file-flush
+group: api
+category: runtime
+update-time: 20260512
+description: Flush the file sink behind a configured runtime logger when one is present.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-flush
+
+Flush the file sink behind a `ConfiguredLogger`. This helper is the file-specific runtime flush surface for config-built file loggers.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_flush(self : ConfiguredLogger) -> Bool {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose file sink should be flushed.
+
+#### output
+
+- `Bool` - Whether the underlying file flush succeeded.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain file sinks forward directly to file flush behavior.
+- Queued file sinks first flush queued records, then flush the wrapped file sink.
+- Non-file sinks return `false`.
+- This helper is narrower than generic `flush()` because it targets file sink behavior specifically.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Explicit File Durability Steps
+
+When a config-built file logger should flush to disk explicitly:
+```moonbit
+ignore(logger.file_flush())
+```
+
+In this example, the file-backed runtime sink is flushed directly through the configured logger.
+
+#### When Need A File-specific Success Flag
+
+When code should branch on the outcome of a file flush:
+```moonbit
+let ok = logger.file_flush()
+```
+
+In this example, callers can distinguish file flush success from a non-file sink shape.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the method returns `false`.
+
+- If callers want generic queue or sink advancement instead of file-specific behavior, `flush()` is the broader API.
+
+### Notes
+
+1. Prefer this helper when the configured sink is known to be file-backed.
+
+2. Queued file sinks may perform both queue flush and file flush work here.