Clarify breaking change process

What does this MR do and why?

It adds clarification around the breaking change process.

According to our documentation, when a feature flag is globally disabled

Features that are disabled by default may change or be removed without notice in a future version of GitLab.

Features behind default-disabled feature flags are not recommended for use in a production environment and problems caused by using a default disabled features aren’t covered by GitLab Support.

In that cases, the breaking change approval should not be requiered.

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