Document that _PyFunction_FastCallDict() must copy kwargs

Issue #29318: Caller and callee functions must not share the dictionary:
kwargs must be copied.

Python/ceval.c

@@ -5039,6 +5039,8 @@ _PyFunction_FastCallDict(PyObject *func, PyObject **args, Py_ssize_t nargs,
     if (nk != 0) {
         Py_ssize_t pos, i;
 
+        /* Issue #29318: Caller and callee functions must not share the
+           dictionary: kwargs must be copied. */
         kwtuple = PyTuple_New(2 * nk);
         if (kwtuple == NULL) {
             return NULL;