API specs in llms.txt

[ethanpalm]

Documentation changes

This PR updates the llms.txt page with the info that API specs are included in the files.

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
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Note

Low Risk
Docs-only wording and example updates; no runtime or API behavior changes.

Overview
Updates the llms.txt documentation to clarify that generated outputs can include API specification links (OpenAPI/AsyncAPI) and that API reference page descriptions incorporate openapi/api frontmatter data, with an expanded example showing an API specifications section.

Also tweaks wording around llms-full.txt indexing and how deleting custom llms.txt/llms-full.txt restores the autogenerated files, and adds llms to the Vale accepted vocabulary to avoid lint errors.

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