Docs: Clarify private CodeQuality runner builds directory configuration

What does this MR do?

This MR clarifies what is required configuration for the builds directory in the private runner configuration and what is optional.

Someone pointed out that the entire exposure of the builds directory is currently described as optional, and that isn't true. We originally described the exposure of the builds directory as required and the setting of the particular builds directory as optional, but they accidentally condensed into a single point at some point in the review process.

Related issues

#267930 (closed) & !49788 (merged)

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• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. I (@drew) wrote this original documentation 👍

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