Clarify re-entrant groupby key comparison comment

Author: VanshAgarwal24036

Modules/itertoolsmodule.c

@@ -545,11 +545,14 @@ groupby_next(PyObject *op)
             break;
         else {
             int rcmp;
+
+            /* A user-defined __eq__ can re-enter groupby and advance the iterator,
+            mutating gbo->tgtkey / gbo->currkey while we are comparing them.
+            Take local snapshots and hold strong references so INCREF/DECREF
+            apply to the same objects even under re-entrancy. */
             PyObject *tgtkey = gbo->tgtkey;
             PyObject *currkey = gbo->currkey;
 
-            /* Hold strong references during comparison to prevent re-entrant __eq__
-            from advancing the iterator and invalidating borrowed references. */
             Py_INCREF(tgtkey);
             Py_INCREF(currkey);