Add more docs for regex

[xu-song]

Purpose

rust style regex is a bit different from python regex. For example, ^(?!abc) is not supported in rust regex.

error: look-around, including look-ahead and look-behind, is not supported

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request adds a note to the documentation specifying that regex patterns for structured outputs should follow Rust style. While this is true for several backends, it's not universally correct, as the lm-format-enforcer backend uses Python's re module. I've added a comment to suggest a more precise clarification to prevent misleading users and causing potential runtime errors.

[mergify]

Documentation preview: https://vllm--30106.org.readthedocs.build/en/30106/