Add missing validations showcase to checkboxes examples

[revolter]

Why:

To prevent incorrect interpretations of the descriptions of the YAML structure required for checkboxes (like in github#41190).

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

All the other types both explain the validations key and then showcase it, but the checkboxes one does not showcase it.

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