Better documentation for InstanceGroups

[Cryptophobia]

InstanceGroups are important and they must be defined. When there is only one availability zone in a region (eu-cental-1) and it has multiple masters, there must be multiple InstanceGroups defined so that the masters will all get configured in etcd and Route53. Otherwise, you can never have more than one master in a single availability zone in a region.

kubernetes/kubernetes#50622

We need better disclaimer/documentation for this...

I can either write some or someone else, but would like it to be peer reviewed because still don't understand kops completely.

[robinpercy]

@Cryptophobia contributions are always welcome - and also always peer reviewed :) When you have something for review, just submit a PR and preface the title with "WIP:" so folks know it's still in progress.

[Cryptophobia]

I opened a PR with some of the text of the disclaimer I want to add to the docs. Would like for other people to peer review as well.

#3445

Thanks!