docs: Note showLastUpdateTime requires complete checkout

[mjec]

Documenting per discussion at #9734

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• N/A If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• N/A If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This requirement isn't clearly documented, but comes up from time to time (e.g. #9734).

Test Plan

Deploy preview: https://deploy-preview-9784--docusaurus-2.netlify.app/docs/api/plugins/@docusaurus/plugin-content-docs/#showLastUpdateTime (direct link to change)

Test links

N/A

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 57	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 87	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 70	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 64	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 89	Report