[EKS addon] [CoreDNS] [request]: Better documentation around tolerations (and potentially others) in ConfigurationValues

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**Tell us about your request**
What do you want us to build? Please improve the documentation around providing custom ConfigurationValues for the CoreDNS Addon. More descriptive error handling would also be very helpful.

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

When adding a taint to the node groups on an EKS cluster for the EBS CSI Driver, as recommended by https://github.com/kubernetes-sigs/aws-ebs-csi-driver/blob/master/docs/install.md#configure-node-startup-taint, a toleration should be added to CoreDNS as it is not dependent on the driver. If CoreDNS is installed using an addon, the toleration must be specified in the ConfigurationValues attribute in either the console or API. The fact that if you do specify additional tolerations, the default tolerations must also be specified or otherwise the user receives a very misleading error message about either schema validation failure, which is not actually the case. Please improve the error messaging and documentation for this and other similar scenarios. Better yet, would be to identify that these tolerations are missing and add them for the user silently, or perhaps with an informational message.

**Are you currently working around this issue?**

Through trial and error and searching through the original helm charts, I was able to determine that the defaults needed also specified however I expect that other users may not have the patience to figure this out.

**Additional context**

**Attachments**

Start with the CoreDNS Addon ConfigurationValues behavior and the linked aws-ebs-csi-driver install.md section on node startup taints. Review how custom tolerations and the default tolerations are documented and how the console or API reports schema validation errors. Done means the documentation explains the required values and the resulting error is no longer misleading.

Written by the indexing model from the issue text.