gh-150232: Change Thread group parameter doc from 'should be' to 'must be'

My-ABC · My-ABC · commit 1ea8974261a6 · 2026-05-22T19:20:04.000+08:00
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
@@ -512,7 +512,7 @@ since it is impossible to detect the termination of alien threads.
    This constructor should always be called with keyword arguments.  Arguments
    are:
 
-   *group* should be ``None``; reserved for future extension when a
+   *group* **must** be ``None``; reserved for future extension when a
    :class:`!ThreadGroup` class is implemented.
 
    *target* is the callable object to be invoked by the :meth:`run` method.
