This was removed in c9bbddcfcb as it was more complicated for operators to figure out _exactly_ which combination of limits/reservations would apply to their applications. Unfortunately, the documentation was never updated.
We may re-add this functionality in the future, but for now, cleaning up the docs is the best option.
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Global limits make it harder to reason about the inheritance in the system. Rather than support global limits, Dokku should provide primitives to automatically set them on an app at provision time, or explicitly set them for an app. Both of these are likely best suited for a higher-level provisioner, or a community plugin.
