Update public-buckets.mdx with partial zone restriction

[kszarlej]

Summary

It seems that R2 custom domain doesn't work when the record is created within a zone that uses partial DNS setup.

Tested using the web console and using the r2_bucket_custom_domain Terraform resource. The error in both cases is non-descriptive - Terraform fails with saying that zone_id is incorrect, and webconsole configures the domain, shows it as green in the R2 bucket settings, but it doesn't work.

On a non-partial domain both TF resource and webconsole works. I used a subdomain delegation setup

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PR Change Summary

Clarified restrictions on using custom domains with R2 buckets.

• Added a restriction stating that the zone containing an R2 Custom Domain must not use a partial DNS setup.

Modified Files

• src/content/docs/r2/buckets/public-buckets.mdx

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[Oxyjun]

Thanks for this PR @kszarlej :D

I realised we're running into a merge conflict, so I've spun another PR to add in the change you're trying to make.

Now tracking here #22011