gh-141617: clarify concurrent.futures.ThreadPoolExecutor deadlock example (GH-141620)

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(cherry picked from commit 171133aa84cd2fa8738bdbb0c76435645810e8d3)

Co-authored-by: Yashraj <yashrajpala8@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: 2 people

Doc/library/concurrent.futures.rst

@@ -156,7 +156,9 @@ And::
        print(f.result())
 
    executor = ThreadPoolExecutor(max_workers=1)
-   executor.submit(wait_on_future)
+   future = executor.submit(wait_on_future)
+   # Note: calling future.result() would also cause a deadlock because
+   # the single worker thread is already waiting for wait_on_future().
 
 
 .. class:: ThreadPoolExecutor(max_workers=None, thread_name_prefix='', initializer=None, initargs=())