Add rate limit note; suggest caching/etags

[jamisonhyatt]

Why:

Context is missing around why PAT is set when fetching GH public keys for verficiationrecommend authentication or caching for secret scanning partners validating the GH pub keys.

Closes [issue link]

What's being changed:

docs for secret scanning

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• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

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[greysteil]

Not sure if we ever refer to personal access tokens as PATs in the docs, but 👍 on the content.

[janiceilene]

Thanks for opening a PR with this addition @jamisonhyatt and thanks for the technical review @greysteil ⚡ I'll get this triaged for review!

[shati-patel]

Thank you for the change @jamisonhyatt 😃

I've added a commit to make the language/style more similar to our existing style, but I'll get this merged afterwards!

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