Document OpenAPI spec inclusion in llms.txt

[mintlify]

Summary

• Updated llms.txt documentation to mention that API reference pages include OpenAPI specification information in their descriptions
• Applied changes to all language versions (en, es, fr, zh)

This reflects the server change in PR #3763 that adds OpenAPI spec info to the llms-full.txt page descriptions.

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Note

Low Risk
Docs-only wording update across multiple locales; no runtime/code behavior changes.

Overview
Updates the llms.txt documentation to clarify that API reference page entries include OpenAPI spec details sourced from openapi/api frontmatter (in addition to description).

Applies the same clarification across the English, Spanish, French, and Chinese versions of the page.

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[ethanpalm]

closing in favor of #3709