Note that the cuDNN version must match exactly on Windows (#10653)

Fixes #10594.

tensorflow/docs_src/install/install_windows.md

@@ -38,9 +38,10 @@ installed on your system:
     [NVIDIA documentation](https://developer.nvidia.com/cuda-gpus) for a
     list of supported GPU cards.
 
-If you have an earlier version of the preceding packages, please
-upgrade to the specified versions.
-
+If you have a different version of one of the preceding packages, please
+change to the specified versions.  In particular, the cuDNN version
+must match exactly: TensorFlow will not load if it cannot find `cuDNN64_5.dll`.
+To use a different version of cuDNN, you must build from source.
 
 ## Determine how to install TensorFlow