📝 Update More API Document

docs/api/split-sink.md

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+---
+name: split-sink
+group: api
+category: sink
+update-time: 20260520
+description: Create a sink that routes records to one of two sinks using a predicate.
+key-word:
+    - sink
+    - split
+    - routing
+    - public
+---
+
+## Split-sink
+
+Create a sink that routes each record to one of two sinks based on a predicate. This helper is useful for target-based or policy-based sink routing.
+
+### Interface
+
+```moonbit
+pub fn[A, B] split_sink(left : A, right : B, predicate : (Record) -> Bool) -> SplitSink[A, B] {
+```
+
+#### input
+
+- `left : A` - Sink receiving records when the predicate returns `true`.
+- `right : B` - Sink receiving records when the predicate returns `false`.
+- `predicate : (Record) -> Bool` - Routing function evaluated for each record.
+
+#### output
+
+- `SplitSink[A, B]` - Conditional routing sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The predicate decides which side receives each record.
+- Unlike `fanout_sink(...)`, a record is routed to one side only.
+- This helper is useful for target-aware and content-aware routing policies.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Predicate-based Routing
+
+When audit records should go to a different destination:
+```moonbit
+let sink = split_sink(console_sink(), json_console_sink(), fn(rec) {
+  rec.target == "audit"
+})
+```
+
+In this example, only records matching the predicate go to the left sink.
+
+### Error Case
+
+e.g.:
+- If both destinations should always receive the same record, use `fanout_sink(...)` instead.
+
+- Predicate logic is caller-defined, so misrouting comes from predicate choice rather than sink mechanics.
+
+### Notes
+
+1. This helper is a routing primitive for synchronous sink composition.
+
+2. `split_by_level(...)` is a convenience wrapper for level-based routing.