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.

Author: pablogsal

Doc/c-api/init.rst

@@ -856,6 +856,12 @@ code, or when embedding the Python interpreter:
    created, the current thread must not have acquired it, otherwise deadlock
    ensues.
 
+   .. note::
+      Calling this function from a thread when the runtime is finalizing
+      will terminate the thread, even if the thread was not created by Python.
+      You can use :c:func:`_Py_IsFinalizing` or :func:`sys.is_finalizing` to
+      check if the interpreter is in process of being finalized before calling
+      this function to avoid unwanted termination.
 
 .. c:function:: PyThreadState* PyThreadState_Get()
 
@@ -903,6 +909,12 @@ with sub-interpreters:
    When the function returns, the current thread will hold the GIL and be able
    to call arbitrary Python code.  Failure is a fatal error.
 
+   .. note::
+      Calling this function from a thread when the runtime is finalizing
+      will terminate the thread, even if the thread was not created by Python.
+      You can use :c:func:`_Py_IsFinalizing` or :func:`sys.is_finalizing` to
+      check if the interpreter is in process of being finalized before calling
+      this function to avoid unwanted termination.
 
 .. c:function:: void PyGILState_Release(PyGILState_STATE)