Add a note explaining why dict_update() doesn't use METH_FASTCALL

Issue #29312.

Objects/dictobject.c

@@ -2347,6 +2347,9 @@ dict_update_common(PyObject *self, PyObject *args, PyObject *kwds,
     return result;
 }
 
+/* Note: dict.update() uses the METH_VARARGS|METH_KEYWORDS calling convention.
+   Using METH_FASTCALL would make dict.update(**dict2) calls slower, see the
+   issue #29312. */
 static PyObject *
 dict_update(PyObject *self, PyObject *args, PyObject *kwds)
 {