pythongh-102304: Add a What's New Entry About _Py_RefTotal (pythongh-102845)

python#102304

Author: ericsnowcurrently

Doc/whatsnew/3.12.rst

@@ -1036,6 +1036,11 @@ Porting to Python 3.12
   functions that set the error indicator now normalize the exception
   before storing it. (Contributed by Mark Shannon in :gh:`101578`.)
 
+* ``_Py_RefTotal`` is no longer authoritative and only kept around
+  for ABI compabitility.  Note that it is an internal global and only
+  available on debug builds.  If you happen to be using it then you'll
+  need to start using ``_Py_GetGlobalRefTotal()``.
+
 Deprecated
 ----------