[Docs] PackageRepository installation removes Packages defined namespace

There's no clear documentation that defined the behavior of kapp-controller when reconciling packages and packagemetadatas from a PackageRepository as it removes the defined namespace from those resources so that they are applied in the same namespace as the packagerepository.

k8s Slack conversation that triggers this issue with all the context: https://kubernetes.slack.com/archives/C02GY94A8KT/p1634834194017300

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