Docs: define what "verified" means for the domain name used in mapping to Cloud Run

[neilstuartcraig]

This is a really simple one....In the Terraform provider docs, the name field is descrived as (Required) Name should be a verified domain but there's no mention or link to what verified means in this context. I assume it means https://support.google.com/webmasters/answer/9008080?hl=en#zippy= but I am not sure. Perhaps the word "verified" could link to that doc (or whichever is relevant).

[github-actions]

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