Add documentation for AioS3FileSystem and S3Executor

Add API reference entries for AioS3FileSystem, AioS3File, S3Executor,
S3ThreadPoolExecutor, and S3AioExecutor to docs/api/filesystem.rst.
Add user guide section to docs/aio.md explaining AioS3FileSystem's
asyncio-native parallel operations, the executor strategy pattern,
and usage examples.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: laughingman7743

docs/aio.md

@@ -193,3 +193,78 @@ The `as_pandas()`, `as_arrow()`, and `as_polars()` convenience methods operate o
 already-loaded data and remain synchronous.
 
 See each cursor's documentation page for detailed usage examples.
+
+(aio-s3-filesystem)=
+
+## AioS3FileSystem
+
+`AioS3FileSystem` is a native asyncio filesystem interface for Amazon S3, built on
+fsspec's `AsyncFileSystem`. It provides the same functionality as `S3FileSystem` but
+uses `asyncio.gather` with `asyncio.to_thread` for parallel operations instead of
+`ThreadPoolExecutor`.
+
+### Why AioS3FileSystem?
+
+The synchronous `S3FileSystem` uses `ThreadPoolExecutor` for parallel S3 operations
+(batch deletes, multipart uploads, range reads). When used from within an asyncio
+application via `AioS3FSCursor`, this creates a thread-in-thread pattern:
+the cursor wraps calls in `asyncio.to_thread()`, and inside that thread
+`S3FileSystem` spawns additional threads via `ThreadPoolExecutor`.
+
+`AioS3FileSystem` eliminates this inefficiency by dispatching all parallel
+operations through the asyncio event loop.
+
+| | S3FileSystem | AioS3FileSystem |
+|---|---|---|
+| **Parallelism** | `ThreadPoolExecutor` | `asyncio.gather` + `asyncio.to_thread` |
+| **File handles** | `S3File` with thread pool | `AioS3File` with `S3AioExecutor` |
+| **Bulk delete** | Thread pool per batch | `asyncio.gather` per batch |
+| **Multipart copy** | Thread pool per part | `asyncio.gather` per part |
+| **Best for** | Synchronous applications | Async frameworks (FastAPI, aiohttp, etc.) |
+
+### Executor strategy
+
+`S3FileSystem` and `S3File` use a pluggable executor abstraction (`S3Executor`) for
+parallel operations. Two implementations are provided:
+
+- `S3ThreadPoolExecutor` — wraps `ThreadPoolExecutor` (default for sync usage)
+- `S3AioExecutor` — dispatches work via `asyncio.run_coroutine_threadsafe` + `asyncio.to_thread`
+
+`AioS3FileSystem` automatically uses `S3AioExecutor` for file handles, so multipart
+uploads and parallel range reads are executed on the event loop without spawning
+additional threads.
+
+### Usage with AioS3FSCursor
+
+`AioS3FSCursor` automatically uses `AioS3FileSystem` internally. No additional
+configuration is needed:
+
+```python
+from pyathena import aio_connect
+from pyathena.aio.s3fs.cursor import AioS3FSCursor
+
+async with await aio_connect(s3_staging_dir="s3://YOUR_S3_BUCKET/path/to/",
+                          region_name="us-west-2") as conn:
+    cursor = conn.cursor(AioS3FSCursor)
+    await cursor.execute("SELECT * FROM many_rows")
+    async for row in cursor:
+        print(row)
+```
+
+### Standalone usage
+
+`AioS3FileSystem` can also be used directly for S3 operations:
+
+```python
+from pyathena.filesystem.s3_async import AioS3FileSystem
+
+# Async context
+fs = AioS3FileSystem(asynchronous=True)
+
+files = await fs._ls("s3://my-bucket/data/")
+data = await fs._cat_file("s3://my-bucket/data/file.csv")
+await fs._rm("s3://my-bucket/data/old/", recursive=True)
+
+# Sync wrappers are auto-generated by fsspec
+files = fs.ls("s3://my-bucket/data/")
+```

docs/api/filesystem.rst

@@ -14,6 +14,27 @@ S3 FileSystem
 .. autoclass:: pyathena.filesystem.s3.S3File
    :members:
 
+Async S3 FileSystem
+-------------------
+
+.. autoclass:: pyathena.filesystem.s3_async.AioS3FileSystem
+   :members:
+
+.. autoclass:: pyathena.filesystem.s3_async.AioS3File
+   :members:
+
+S3 Executor
+-----------
+
+.. autoclass:: pyathena.filesystem.s3_executor.S3Executor
+   :members:
+
+.. autoclass:: pyathena.filesystem.s3_executor.S3ThreadPoolExecutor
+   :members:
+
+.. autoclass:: pyathena.filesystem.s3_executor.S3AioExecutor
+   :members:
+
 S3 Objects
 ----------