[EKS] [request]: Improve managed node group update behaviour documentation

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**Tell us about your request**
I'd like to see the [Managed node update behavior](https://docs.aws.amazon.com/eks/latest/userguide/managed-node-update-behavior.html) documentation improved. The current documentation doesn't seem correct if the max unavailable isn't 100% as there is no documented connection between the partially completed upgrade phase back to the scale up phase. It would also be useful to describe some actual considerations as to why you'd choose one pattern over another. This would also be relevant in the [EKS Best Practices Guides](https://aws.github.io/aws-eks-best-practices/) which currently promotes MNGs without any actual content describing them or why you'd actually want to use them (or not).

**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?**
I'd like to be able to have a high availability set of MNGs that can be updated without having un-schedulable pods. This is specifically relevant where you're using CA, PVs and so have a MNG per AZ.

**Are you currently working around this issue?**
Setting `maxUnavailable` to `1` and hoping it works.

**Additional context**
n/a

**Attachments**
n/a

Start with the linked Managed node update behavior documentation and the EKS Best Practices Guides. Compare the documented behavior when maxUnavailable is below 100%, then update the guidance to explain the upgrade phases and the considerations for choosing different patterns, including high availability across AZs.

Written by the indexing model from the issue text.