bpo-38266: Revert bpo-37878: Make PyThreadState_DeleteCurrent() Internal (GH-16558)

Revert the removal of PyThreadState_DeleteCurrent() with documentation.
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index f872351..6c8d2ae 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -183,6 +183,7 @@
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Next(PyInterpreterState *);
 PyAPI_FUNC(PyThreadState *) PyInterpreterState_ThreadHead(PyInterpreterState *);
 PyAPI_FUNC(PyThreadState *) PyThreadState_Next(PyThreadState *);
+PyAPI_FUNC(void) PyThreadState_DeleteCurrent(void);
 
 typedef struct _frame *(*PyThreadFrameGetter)(PyThreadState *self_);