📝 document buffered sink methods

docs/api/buffered-sink-pending-count.md

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+---
+name: buffered-sink-pending-count
+group: api
+category: sink
+update-time: 20260613
+description: Read the current buffered-record count from a BufferedSink.
+key-word:
+    - sink
+    - buffer
+    - queue
+    - public
+---
+
+## Buffered-sink-pending-count
+
+Read the current buffered-record count from a `BufferedSink[S]`. This is the direct sink-level metric for how many records are still waiting in the in-memory buffer.
+
+### Interface
+
+```moonbit
+pub fn[S] BufferedSink::pending_count(self : BufferedSink[S]) -> Int {
+```
+
+#### input
+
+- `self : BufferedSink[S]` - Buffered sink whose current in-memory backlog should be inspected.
+
+#### output
+
+- `Int` - Current number of buffered records.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The return value is `self.buffer.val.length()` at the time of the call.
+- This is a point-in-time metric and may change immediately after it is read.
+- It reflects buffered records that have not yet been flushed to the wrapped sink.
+- This helper does not mutate the sink.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Direct Buffer Backlog Visibility
+
+When code is working with a `BufferedSink` value directly and wants to observe pending buffered records:
+```moonbit
+let pending = sink.pending_count()
+```
+
+In this example, callers can inspect the current in-memory backlog without touching the wrapped sink.
+
+#### When Verify Manual Flush Progress
+
+When explicit flush steps should be checked operationally:
+```moonbit
+ignore(sink.flush())
+ignore(sink.pending_count())
+```
+
+In this example, the metric helps verify whether buffered records were forwarded out of the local buffer.
+
+### Error Case
+
+e.g.:
+- This helper does not have a normal failure mode; it only reads current buffer length.
+
+- If callers need overflow-aware backlog semantics instead of simple buffering, `QueuedSink::pending_count()` is the better API.
+
+### Notes
+
+1. Use this helper for direct visibility into simple synchronous buffering.
+
+2. Pair it with `flush()` when inspecting whether batched writes have been forwarded.