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Update GPU bots documentation.

Bug: 962876
Change-Id: I3d4ff17f242eb1e481bd4dddfc82f8a2914f521d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040136
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739446}