📝 Update More API Document

docs/api/filter-sink.md

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+---
+name: filter-sink
+group: api
+category: sink
+update-time: 20260520
+description: Create a sink that forwards only records matching a predicate.
+key-word:
+    - sink
+    - filter
+    - predicate
+    - public
+---
+
+## Filter-sink
+
+Create a sink that forwards only records matching a predicate. This helper is the sink-level counterpart to `Logger::with_filter(...)`.
+
+### Interface
+
+```moonbit
+pub fn[S] filter_sink(sink : S, predicate : (Record) -> Bool) -> FilterSink[S] {
+```
+
+#### input
+
+- `sink : S` - Wrapped sink receiving records that pass the predicate.
+- `predicate : (Record) -> Bool` - Filtering function evaluated for each record.
+
+#### output
+
+- `FilterSink[S]` - Predicate-filtering sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Only records for which the predicate returns `true` are forwarded.
+- This helper is useful for sink-first composition and explicit routing graphs.
+- Use the logger-level helper when the filter should be attached after `Logger::new(...)` instead.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Predicate Filtering At Sink Construction Time
+
+When a sink graph should reject records before they reach the destination:
+```moonbit
+let sink = filter_sink(console_sink(), fn(rec) {
+  rec.target == "kept"
+})
+```
+
+In this example, only matching records are forwarded to the wrapped sink.
+
+### Error Case
+
+e.g.:
+- If record selection should be attached to an already-built logger, use `with_filter(...)` instead.
+
+- Predicate logic is caller-defined, so accidental over-filtering comes from predicate choice.
+
+### Notes
+
+1. This helper is useful for sink-first composition graphs.
+
+2. It works well with predicate builders such as `target_is(...)` and `all_of(...)`.