Update Docs for Calico Backend for kops 1.12

[gjtempleton]

Current docs still state that Calico uses etcd as the backend, makes clear that this only holds true for versions below 1.12 and adds a stanza around using APIServer without Typha for 1.12 onwards.

See: https://github.com/kubernetes/kops/blob/master/upup/models/cloudup/resources/addons/networking.projectcalico.org/k8s-1.12.yaml.template#L187

Minor markdown cleanup in the surrounding areas as well.

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[justinsb]

Thanks @gjtempleton

/approve
/lgtm
/ok-to-test

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gjtempleton, justinsb

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[k8s-ci-robot]

@gjtempleton: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kops-e2e-kubernetes-aws	a517fcd	link	/test pull-kops-e2e-kubernetes-aws

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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