Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Improve documentation for kubeadm phases
**What this PR does / why we need it**:
This PR is part of the effort for improving kubeadm reference doc (CLI, website, manpages), and more specifically improves documentation for:
- kubeadm alpha phase certs
- kubeadm alpha phase kubeconfig
- kubeadm alpha phase controlplane
- kubeadm alpha phase etcd
- kubeadm alpha phase uploadconfig
- kubeadm alpha phase addons
- kubeadm alpha phase selfhosting
**Which issue(s) this PR fixes**:
part of the effort for [#265](https://github.com/kubernetes/kubeadm/issues/265)
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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