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Add instructions on how to debug a crashed pod

When I was using `kubeadm init`, I ran into an issue where I had passed an
invalid flag through the kubeadm config file. The flag was being passed into
apiserver and preventing it from launching with a "unknown flag" error.

The flag in question is (other flags elided for clarity):

```
admission-control: ...,GenericAdmissionWebhook,...
```

Since this prevented the apiserver from coming up, the setup timed out
and gave me the error message I just modified.

It would be better if the config was vetted more thoroughly, but I think
documenting the backup strategy for viewing logs in case of failure is
also valuable.



**What this PR does / why we need it**:
This PR extends an error message in kubeadm to make it clear a potential step forward for new cluster users.

**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#59731 

**Special notes for your reviewer**:

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