Clarify description truncation in llms.txt

[mintlify-development]

Summary

• Documents that page descriptions in llms.txt are truncated to the first paragraph and limited to 300 characters

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Note

Low Risk
Low risk documentation-only change that clarifies existing llms.txt generation behavior without affecting runtime logic.

Overview
Clarifies in ai/llmstxt.mdx that page descriptions included in generated llms.txt entries are truncated to 300 characters and stop at the first line break, in addition to being sourced from frontmatter (and augmented for API reference pages).

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