📝 document public config and level aliases

docs/api/level.md

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+---
+name: level
+group: api
+category: level
+update-time: 20260613
+description: Public level enum alias shared by records, filters, and logger thresholds.
+key-word:
+    - level
+    - alias
+    - severity
+    - public
+---
+
+## Level
+
+`Level` is the public severity enum used across records, filters, and logger threshold checks. It is a direct alias to the core level type, so the same five built-in variants are shared everywhere the public API accepts or returns a level.
+
+### Interface
+
+```moonbit
+pub type Level = @core.Level
+```
+
+#### output
+
+- `Level` - Public severity enum with the variants `Trace`, `Debug`, `Info`, `Warn`, and `Error`.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a type alias, not a wrapper or a separate enum.
+- The built-in variants are `Level::Trace`, `Level::Debug`, `Level::Info`, `Level::Warn`, and `Level::Error`.
+- `Level` is used by `Record::new(...)`, logger threshold APIs, predicate helpers, and config-driven builders.
+- Ordering and display behavior come from the same underlying type, so helpers such as `priority()`, `label()`, and `enabled(...)` work on this alias unchanged.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Severity For New Records
+
+When a record should be created with a concrete severity:
+```moonbit
+let rec = Record::new(Level::Warn, "disk almost full")
+```
+
+In this example, the record uses the public `Level` alias directly.
+
+#### When Need A Threshold For Logger Filtering
+
+When a logger should only accept records at or above one severity:
+```moonbit
+let logger = Logger::new(console_sink()).with_min_level(Level::Info)
+```
+
+In this example, the same public level enum is reused as the threshold value.
+
+### Error Case
+
+e.g.:
+- `Level` itself does not have a runtime failure mode.
+
+- If external config text names an unsupported level string, parsing fails through `ConfigError` instead of producing a fallback level.
+
+### Notes
+
+1. Use `label()` for display text and `priority()` or `enabled(...)` for threshold logic.
+
+2. This alias keeps sync APIs, async APIs, and config builders on the same severity vocabulary.