Add missing scopes to the personal access tokens API docs

What does this MR do and why?

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It was pointed out at https://github.com/gitlabhq/terraform-provider-gitlab/pull/1007#discussion_r841251373 that our API documentation for personal access tokens does not document the read_registry, write_registry, or sudo scopes as valid parameters. These already exist as valid parameters implementation-wise, but they aren't documented. This MR adds them to the documentation.

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