Document helm template name and namespace annotations

There's 2 experimental annotations that helps helmTemplate processing to define the name and namespace, but these are not documented:
https://github.com/vmware-tanzu/carvel-kapp-controller/blob/22d3f3a23048f41b9dc7e9c56944ef67c2819758/pkg/packageinstall/app.go#L24-L25

They should be in the docs.

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