📝 Update More API Document

docs/api/callback-sink.md

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+---
+name: callback-sink
+group: api
+category: sink
+update-time: 20260520
+description: Create a sink that forwards records to a user callback.
+key-word:
+    - sink
+    - callback
+    - record
+    - public
+---
+
+## Callback-sink
+
+Create a sink that forwards each `Record` to a callback. This is the most direct built-in integration hook for tests, adapters, and custom side effects.
+
+### Interface
+
+```moonbit
+pub fn callback_sink(callback : (Record) -> Unit) -> CallbackSink {
+```
+
+#### input
+
+- `callback : (Record) -> Unit` - Function called for each emitted record.
+
+#### output
+
+- `CallbackSink` - Sink that forwards records to the callback.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The callback receives the full structured record.
+- This sink is useful for tests, custom bridges, or integration code that wants raw record access.
+- Formatting is not applied automatically because the callback works on `Record` values directly.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Capture Structured Records
+
+When tests or adapters want direct access to target, message, and fields:
+```moonbit
+let logger = Logger::new(
+  callback_sink(fn(rec) {
+    println(rec.target)
+  }),
+  target="hook",
+)
+```
+
+In this example, the callback sees the structured record rather than pre-rendered text.
+
+### Error Case
+
+e.g.:
+- If text output is needed instead of raw records, use `text_callback_sink(...)`.
+
+- Callback behavior is fully user-defined, so failures inside the callback are outside the sink's own API contract.
+
+### Notes
+
+1. This sink is commonly useful in tests and adapters.
+
+2. It composes naturally with filter, patch, fanout, and queue wrappers.