admission-controller-webhook-replacement check needs more details

This check just outputs that the webhook is configured incorrectly and can cause problems with DOKS cluster upgrades. But, adding in more details as to how the webhook is configured wrong or what config changes can be done to fix it will help users more.

Context: https://kubernetes.slack.com/archives/CCPETNUCA/p1598549516051000

This check can also use more test cases that can cause it to fail. This also serves as documentation for anyone trying to understand how their webhook config can cause the check to fail.

Locate the admission-controller-webhook-replacement check and its existing tests, then read how the current diagnostic is generated. Add details explaining the misconfiguration and possible fixes, and add test cases for webhook configurations that trigger the check; done means the expanded output and cases clearly document the failure modes.

Written by the indexing model from the issue text.