Fix asyncio.to_thread() documented return type (GH-20547)

Fix asyncio.to_thread() documented return type (GH-20547)

When I wrote the documentation for `asyncio.to_thread()`, I mistakenly assumed that `return await loop.run_in_executor(...)` within an async def function would return a Future. In reality, it returns a coroutine.

This likely won't affect typical usage of `asyncio.to_thread()`, but it's important for the documentation to be correct here. In general, we also tend to avoid returning futures from high-level APIs in asyncio.
(cherry picked from commit 2b201369b435a4266bda5b895e3b615dbe28ea6e)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>