Updates policy command usage and Go docs

General MR

Summary

Updates policy command usage and Go docs.

Fixes an issue found when testing against Dogfood where the Kyverno policy fetch would fail when certain CRDs did not have an implementation for every version of the k8s API.

Adds new unit test to cover the case which fixes the Kyverno policy fetch issue.

Relevant logs/screenshots

Linked Issue

#85 (closed)

Upgrade Notices

N/A

Closes #85 (closed)