Tweak docs about disabling CUDA devices

Author: IanButterworth

docs/src/gpu.md

@@ -90,14 +90,14 @@ julia> x |> cpu
 
 Sometimes it is required to control which GPUs are visible to `julia` on a system with multiple GPUs or disable GPUs entirely. This can be achieved with an environment variable `CUDA_VISIBLE_DEVICES`.
 
+To disable all devices:
+```
+$ export CUDA_VISIBLE_DEVICES='-1'
+```
+To select specific devices by device id:
 ```
-$ export CUDA_VISIBLE_DEVICES=''
-
-# or
-
 $ export CUDA_VISIBLE_DEVICES='0,1'
 ```
 
-The environment variable `CUDA_VISIBLE_DEVICES` can also take a comma separated list of device IDs (`0,1,...`) which would be visible to CUDA related processes.
 
 More information for conditional use of GPUs in CUDA.jl can be found in its [documentation](https://cuda.juliagpu.org/stable/installation/conditional/#Conditional-use), and information about the specific use of the variable is described in the [Nvidia CUDA blogpost](https://developer.nvidia.com/blog/cuda-pro-tip-control-gpu-visibility-cuda_visible_devices/).