docs(dns): Clarify that the ACL field in DNS zone resource has no effect

[ondbeh]

relates to STACKITDNS-980

Description

The ACL field in the POST and PATCH zone endpoints of our API and in the terraform provider implementation has currently no effect. I have updated the documentation for this field to acknowledge this and avoid any confusion.

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relates to STACKITDNS-980

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)