📝 document buffered sink methods

docs/api/buffered-sink-flush.md

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+---
+name: buffered-sink-flush
+group: api
+category: sink
+update-time: 20260613
+description: Flush buffered records from a BufferedSink into its wrapped sink.
+key-word:
+    - sink
+    - buffer
+    - flush
+    - public
+---
+
+## Buffered-sink-flush
+
+Flush buffered records from a `BufferedSink[S]` into its wrapped sink. This is the direct sink-level batching control API for simple synchronous buffering.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] BufferedSink::flush(self : BufferedSink[S]) -> Unit {
+```
+
+#### input
+
+- `self : BufferedSink[S]` - Buffered sink whose pending records should be forwarded to the wrapped sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- If the buffer is empty, the method does nothing.
+- When buffered records exist, they are copied into a temporary `pending` array, the live buffer is cleared, and each record is then written to the wrapped sink in order.
+- This helper drains only the local buffer; any later behavior still depends on the wrapped sink `S`.
+- The method does not return a count or success flag.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Explicit Batch Delivery
+
+When a buffered sink should forward pending records before the threshold is reached:
+```moonbit
+sink.flush()
+```
+
+In this example, callers force buffered records to be written to the wrapped sink immediately.
+
+#### When Need A Manual Flush Barrier
+
+When tests or synchronous code want buffering but still need a deliberate release point:
+```moonbit
+let sink = buffered_sink(console_sink(), flush_limit=4)
+sink.flush()
+```
+
+In this example, the buffered sink exposes a direct batching barrier without using queue semantics.
+
+### Error Case
+
+e.g.:
+- If the wrapped sink's write behavior fails or has side effects, this helper does not add an extra reporting layer on top of `S`.
+
+- If callers need a count-returning drain API with drop tracking, `QueuedSink::flush()` or `QueuedSink::drain()` may fit better.
+
+### Notes
+
+1. Use this helper when code owns a `BufferedSink` directly and wants explicit control over when buffered records are forwarded.
+
+2. Automatic flush still happens when buffered record count reaches `flush_limit` during writes.