[Rules]Update fields-functions.mdx

[nenizera]

Hello team ,

Summary

You can't rewrite cdn-cgi

Thanks!

[hyperlint-ai]

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PR Change Summary

Updated documentation to clarify that rewriting the /cdn-cgi/ endpoint is not allowed for security reasons.

• Added a note regarding the restriction on rewriting the /cdn-cgi/ endpoint for security reasons.
• Clarified the usage of specific functions in rewrite expressions.

Modified Files

• src/content/docs/rules/transform/url-rewrite/reference/fields-functions.mdx

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[pedrosousa]

This is being addressed in another PR: #21043.