Update rules breaking change in 13.0: rules and only/except cannot co-exist

Why is this change being made?

The current text is confusing, it might lead to think that rules and only/except can coexist in some "occasional" cases. This is not true, and users MUST update their configuration if they use only/except (https://docs.gitlab.com/ee/ci/yaml/#rules).

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@tmccaslin could you please take a look? Thanks

/cc @theoretick