📝 document wrapper sink types

docs/api/patch-sink-type.md

@@ -0,0 +1,75 @@
+---
+name: patch-sink-type
+group: api
+category: sink
+update-time: 20260613
+description: Public patch sink type used for record-rewriting forwarding over another sink.
+key-word:
+    - sink
+    - patch
+    - type
+    - public
+---
+
+## Patch-sink-type
+
+`PatchSink[S]` is the public record-rewriting sink type used for forwarding patched records to another sink. It is the concrete sink type returned by `patch_sink(...)` and preserves the wrapped sink type in its type parameter.
+
+### Interface
+
+```moonbit
+pub struct PatchSink[S] {
+  sink : S
+  patch : RecordPatch
+}
+```
+
+#### output
+
+- `PatchSink[S]` - Public synchronous sink type that rewrites records before forwarding them to the wrapped sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a public root struct, not a type alias.
+- The current fields are `sink : S` and `patch : RecordPatch`.
+- `patch_sink(...)` constructs this type directly from a wrapped sink and patch function.
+- The sink preserves the wrapped sink type `S`, which is useful when typed composition still matters after record rewriting is introduced.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Typed Record-rewriting Sink Value
+
+When code should keep the concrete patch wrapper sink type visible:
+```moonbit
+let sink : PatchSink[ConsoleSink] = patch_sink(console_sink(), prefix_message("[safe] "))
+```
+
+In this example, the sink value stays explicit and preserves the wrapped console sink type.
+
+#### When Need A Typed Logger With Sink-level Patching
+
+When logging should preserve the patch wrapper sink type in the logger:
+```moonbit
+let logger : Logger[PatchSink[ConsoleSink]] = Logger::new(
+  patch_sink(console_sink(), set_target("audit")),
+)
+```
+
+In this example, the concrete patch sink remains part of the logger type.
+
+### Error Case
+
+e.g.:
+- `PatchSink[S]` itself does not have a runtime failure mode.
+
+- Runtime behavior still depends on the wrapped sink `S`, and semantic rewrite mistakes come from the supplied patch function rather than the type itself.
+
+### Notes
+
+1. Use `patch_sink(...)` when you need a value of this type.
+
+2. Use `FilterSink[S]` when records should be conditionally dropped rather than rewritten.