ggml: add comment with link to discussion suspicious function param

ggml.c

@@ -14755,6 +14755,8 @@ static void ggml_compute_backward(struct ggml_context * ctx, struct ggml_tensor
                     const int n_past = ((int32_t *) tensor->op_params)[0];
                     src0->grad =
                         ggml_add_or_set(ctx, src0->grad,
+                            /* ggml_diag_mask_inf_impl() shouldn't be here */
+                            /* ref:  https://github.com/ggerganov/llama.cpp/pull/4203#discussion_r1412377992 */
                             ggml_diag_mask_zero_impl(ctx, tensor->grad, n_past, false),
                         zero_table);
                 }