docs: Avoid omitempty with kubebuilder:validation:Required #1695
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When writing a unionDiscriminator, use of ",omitempty" in the JSON spec overrides any attempt to require the discriminator via the tag +kubebuilder:validation:Required. It is confusing to have the docs recommend two conflicting pieces of information for the same field, and it makes more sense for the discriminator to be mandatory (even though, as correctly pointed out below, all of the other fields of the union type must be optional).
See also kubernetes-sigs/kubebuilder#3794, where a debate was held on whether kubebuilder should document the interplay, which in turn pointed back to controller-tools conventions.