Mention new C-API in whatsnew

Author: itamaro

Doc/whatsnew/3.15.rst

@@ -950,6 +950,10 @@ New features
 * Add :c:func:`PyTuple_FromArray` to create a :class:`tuple` from an array.
   (Contributed by Victor Stinner in :gh:`111489`.)
 
+* Add a new :c:func:`PyImport_CreateModuleFromInitfunc` C-API for creating
+  a module from a *spec* and *initfunc*.
+  (Contributed by Itamar Oren in :gh:`116146`.)
+
 
 Changed C APIs
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