Clarify what is the default executor for asyncio.run_in_executor

[aisk]

Documentation

The document of asyncio said:

cpython/Doc/library/asyncio-eventloop.rst

Lines 1263 to 1264 in 84512c0

	The *executor* argument should be an :class:`concurrent.futures.Executor`
	instance. The default executor is used if *executor* is ``None``.

But it dosn't mention that what is the default executor, like if it's ThreadPoolExecutor or ProcessPoolExecutor.

Someone on stackoverflow got the same question: https://stackoverflow.com/questions/60204054/default-executor-asyncio

Linked PRs

• gh-121333: Clarify what is the default executor for asyncio.run_in_executor #121335
• [3.12] gh-121333: Clarify what is the default executor for asyncio.run_in_executor (GH-121335) #121525
• [3.13] gh-121333: Clarify what is the default executor for asyncio.run_in_executor (GH-121335) #121526