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

gh-121333: Clarify what is the default executor for asyncio.run_in_executor (GH-121335)
(cherry picked from commit facf986)

Co-authored-by: AN Long <[email protected]>
Co-authored-by: Kumar Aditya <[email protected]>

Author: 3 people

Doc/library/asyncio-eventloop.rst

@@ -1262,6 +1262,9 @@ Executing code in thread or process pools
 
    The *executor* argument should be an :class:`concurrent.futures.Executor`
    instance. The default executor is used if *executor* is ``None``.
+   The default executor can be set by :meth:`loop.set_default_executor`,
+   otherwise, a :class:`concurrent.futures.ThreadPoolExecutor` will be
+   lazy-initialized and used by :func:`run_in_executor` if needed.
 
    Example::