Add warning to PyThreadState_Swap() that it should not be used in Cython code...

Add warning to PyThreadState_Swap() that it should not be used in Cython code because Cython makes the assumption that the current thread state does not change in arbitrary places.

Cython/Includes/cpython/pystate.pxd

@@ -48,7 +48,7 @@ cdef extern from "Python.h":
     void PyThreadState_Delete(PyThreadState *)
 
     PyThreadState * PyThreadState_Get()
-    PyThreadState * PyThreadState_Swap(PyThreadState *)
+    PyThreadState * PyThreadState_Swap(PyThreadState *)  # NOTE: DO NOT USE IN CYTHON CODE !
     PyObject * PyThreadState_GetDict()
     int PyThreadState_SetAsyncExc(long, PyObject *)