Update note about storage on shared GPU runners

What does this MR do?

Update details about the 50GB disk volume on shared GPU runners.

I found this out from the pipeline here, ran on a saas-linux-medium-amd64-gpu-standard runner: https://gitlab.com/gitlab-gold/kenneth-gold/gpu_runner_test/-/jobs/4323152743

In comparison, from a saas-linux-medium-amd64 runner: https://gitlab.com/gitlab-gold/kenneth-gold/gpu_runner_test/-/jobs/4323172610

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