bpo-36427: Document that PyEval_RestoreThread and PyGILState_Ensure can terminate the calling thread (GH-12541)

Calling these function from a thread when the runtime is finalizing will terminate
the thread, even if the thread was not created by Python. Users can use 
_Py_IsFinalizing or sys.is_finalizing to check if the interpreter is in the process of
being finalized before calling this function to avoid unwanted termination.