[Turnstile] Remove duplicate wording

[cicku]

Documentation checklist

• The documentation style guide has been adhered to.

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PR Change Summary

Removed duplicate wording and clarified hostname management details in the documentation.

• Eliminated redundant phrasing regarding hostname limits.
• Clarified availability for Enterprise customers regarding hostname requirements.
• Removed optional hostname validation section to streamline content.

Modified Files

• src/content/docs/turnstile/concepts/hostname-management.mdx

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[patriciasantaana]

Added back the section on optional hostname validation per PM's request.

[cicku]

We should avoid "entitlement" as much as possible, this isn't something should be commonly mentioned, use "feature" instead