gh-85299: Add note warning about entry point guard for asyncio example (GH-93457)

(cherry picked from commit 79fd6cc)

Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>

Author: miss-islington

Doc/library/asyncio-eventloop.rst

@@ -1232,7 +1232,13 @@ Executing code in thread or process pools
                   pool, cpu_bound)
               print('custom process pool', result)
 
-      asyncio.run(main())
+      if __name__ == '__main__':
+          asyncio.run(main())
+
+   Note that the entry point guard (``if __name__ == '__main__'``)
+   is required for option 3 due to the peculiarities of :mod:`multiprocessing`,
+   which is used by :class:`~concurrent.futures.ProcessPoolExecutor`.
+   See :ref:`Safe importing of main module <multiprocessing-safe-main-import>`.
 
    This method returns a :class:`asyncio.Future` object.
 

Doc/library/multiprocessing.rst

@@ -2954,6 +2954,8 @@ Global variables
     However, global variables which are just module level constants cause no
     problems.
 
+.. _multiprocessing-safe-main-import:
+
 Safe importing of main module
 
     Make sure that the main module can be safely imported by a new Python