[Doc] Enhance cuda upgrade doc (#1835)

* Correct cudnn version matrix.

* Add reference

Author: nWEIdia

CUDA_UPGRADE_GUIDE.MD

@@ -5,13 +5,13 @@ So you wanna upgrade PyTorch to support a new CUDA? Follow these steps in order!
 ## 0. Before starting upgrade process
 
 ### A. Currently Supported CUDA and CUDNN Libraries
-Here is the supported matrix for CUDA and CUDNN
+Here is the supported matrix for CUDA and CUDNN (versions can be looked up in https://github.com/pytorch/pytorch/blob/main/.ci/docker/common/install_cudnn.sh)
 
 | CUDA | CUDNN | additional details |
 | --- | --- | --- |
-| 11.8 | 8.9.2.26 | Legacy CUDA Release |
+| 11.8 | 8.7.0.84 | Legacy CUDA Release |
 | 12.1 | 8.9.2.26  | Stable CUDA Release |
-| 12.4 | 8.9.2.26  | Latest CUDA Nightly |
+| 12.4 | 8.9.7.29  | Latest CUDA Nightly |
 
 
 ### B. Check the package availability