Automatic merge from submit-queue (batch tested with PRs 55868, 55393, 55152, 55849). 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 preflight command

**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 preflight`

**Which issue(s) this PR fixes**:
part of the effort for [#265](https://github.com/kubernetes/kubeadm/issues/265)

**Special notes for your reviewer**:
The PR has two commits:
- `Clone documentation utility from //pkg/kubectl/cmd/templates`, that re-implements in kubeadm some kubectl utilities for handling description and examples for cobra.commands; more PR will follow re-using those utilities. NB. I was forced to add an exception to golint because `blackfriday.Renderer` requires to implement an interface that is not compliant
- `kubeadm-doc-preflight` that implements improvement to `kubeadm alpha phase preflight`

**Release note**:
```release-note
NONE
```