Add runAsHostUser docs

[AndrewKahr]

Changes

• Add runAsHostUser docs

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

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[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 9e7f864):

https://game-ci-5559f--pr438-add-runashostuser-vjbfgqd8.web.app

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