Improve threading.daemon docstring (GH-14278)

Rephrase and clarify that "the entire Python program exits when only daemon threads are left". This matches the documentation at https://docs.python.org/3/library/threading.htmlGH-thread-objects.
(cherry picked from commit bb110cc2)
Co-authored-by: mbarkhau <mbarkhau@gmail.com>

Lib/threading.py

@@ -1005,8 +1005,7 @@ class Thread(_Verbose):
         main thread is not a daemon thread and therefore all threads created in
         the main thread default to daemon = False.
 
-        The entire Python program exits when no alive non-daemon threads are
-        left.
+        The entire Python program exits when only daemon threads are left.
 
         """
         assert self.__initialized, "Thread.__init__() not called"