Automatic merge from submit-queue (batch tested with PRs 60200, 63623, 63406). 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>.

kubeadm-init: add details about --token

**What this PR does / why we need it**:
Add details about the required format when passing
a bootstrap token using 'kubeadm init --token'.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes kubernetes/kubeadm#768

**Special notes for your reviewer**:

NONE

**Release note**:

```release-note
NONE
```