📝 Update More API Document

docs/api/split-by-level.md

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+---
+name: split-by-level
+group: api
+category: sink
+update-time: 20260520
+description: Create a split sink that routes records by minimum enabled level.
+key-word:
+    - sink
+    - split
+    - level
+    - public
+---
+
+## Split-by-level
+
+Create a split sink that routes records to one of two sinks based on whether the record level is enabled at a given minimum level.
+
+### Interface
+
+```moonbit
+pub fn[A, B] split_by_level(
+  left : A,
+  right : B,
+  min_level~ : Level = Level::Warn,
+) -> SplitSink[A, B] {
+```
+
+#### input
+
+- `left : A` - Sink receiving records at or above `min_level`.
+- `right : B` - Sink receiving records below `min_level`.
+- `min_level : Level` - Threshold used for routing.
+
+#### output
+
+- `SplitSink[A, B]` - Level-based routing sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a convenience wrapper over `split_sink(...)`.
+- High-severity and low-severity records can be separated without writing the predicate manually.
+- It is useful for patterns such as warnings/errors to one destination and info/debug to another.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Severity-based Routing
+
+When warnings and errors should go to a special path:
+```moonbit
+let sink = split_by_level(callback_sink(fn(rec) {
+  println(rec.message)
+}), console_sink(), min_level=Level::Warn)
+```
+
+In this example, `WARN` and above go to the left sink.
+
+### Error Case
+
+e.g.:
+- If routing should use target, message, or fields instead of level, use `split_sink(...)` with a custom predicate.
+
+- Sink-specific write behavior still follows the wrapped sinks.
+
+### Notes
+
+1. This is a convenience routing helper for a common policy.
+
+2. It preserves the same one-side-only routing model as `split_sink(...)`.