pythongh-99113: Add a What's New Entry for PEP 684 (pythongh-107520)

(cherry picked from commit 79e479c)

Co-authored-by: Eric Snow <[email protected]>

Doc/whatsnew/3.12.rst

@@ -70,6 +70,10 @@ New grammar features:
 
 * :pep:`701`: Syntactic formalization of f-strings
 
+Interpreter improvements:
+
+* :ref:`whatsnew312-pep684`
+
 New typing features:
 
 * :pep:`688`: Making the buffer protocol accessible in Python
@@ -277,6 +281,36 @@ The new :class:`inspect.BufferFlags` enum represents the flags that
 can be used to customize buffer creation.
 (Contributed by Jelle Zijlstra in :gh:`102500`.)
 
+.. _whatsnew312-pep684:
+
+PEP 684: A Per-Interpreter GIL
+------------------------------
+
+Sub-interpreters may now be created with a unique GIL per interpreter.
+This allows Python programs to take full advantage of multiple CPU
+cores.
+
+Use the new :c:func:`Py_NewInterpreterFromConfig` function to
+create an interpreter with its own GIL::
+
+   PyInterpreterConfig config = {
+       .check_multi_interp_extensions = 1,
+       .gil = PyInterpreterConfig_OWN_GIL,
+   };
+   PyThreadState *tstate = NULL;
+   PyStatus status = Py_NewInterpreterFromConfig(&tstate, &config);
+   if (PyStatus_Exception(status)) {
+       return -1;
+   }
+   /* The new interpeter is now active in the current thread. */
+
+For further examples how to use the C-API for sub-interpreters with a
+per-interpreter GIL, see :source:`Modules/_xxsubinterpretersmodule.c`.
+
+A Python API is anticipated for 3.13.  (See :pep:`554`.)
+
+(Contributed by Eric Snow in :gh:`104210`, etc.)
+
 New Features Related to Type Hints
 ==================================
 
@@ -1744,6 +1778,12 @@ New Features
 
   (Contributed by Eddie Elizondo in :gh:`84436`.)
 
+* :pep:`684`: Added the new :c:func:`Py_NewInterpreterFromConfig`
+  function and :c:type:`PyInterpreterConfig`, which may be used
+  to create sub-interpreters with their own GILs.
+  (See :ref:`whatsnew312-pep684` for more info.)
+  (Contributed by Eric Snow in :gh:`104110`.)
+
 * In the limited C API version 3.12, :c:func:`Py_INCREF` and
   :c:func:`Py_DECREF` functions are now implemented as opaque function calls to
   hide implementation details.