Uploaded new patch.

> - the doc addition needs a "versionadded" tag
Fixed.

> - "The main thread is the thread that the OS creates to run application.": I would rephrase this "In normal conditions, the main thread is the thread from which the Python interpreter was started".
Fixed.

> - in the tests:
> +        self.assertEqual(data, "Thread-1\nTrue\nTrue\n")
> 
> Hmm, how do you know it will be called "Thread-1"?
> I would give a specific name to the Thread, so as to make the test deterministic.
In this test main thread after forking is always first thread created by python. That's why it always is called 'Thread-1'. 

> +        self.assertEqual(rc, 0)
> 
> You don't need this, it is already ensured by assert_python_ok().
Fixed.

> - in threading.py, why doesn't _exitfunc() reuse the _main_thread global variable, instead of taking it as a parameter?
Fixed.