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Improve cronjob concurrency policy doc

**What this PR does / why we need it**:
The CronJob API documentation (https://kubernetes.io/docs/api-reference/v1.8/#cronjobspec-v1beta1-batch) did not clearly state what can be used as `concurrencyPolicy`. Instead, it only says that the default value is "Raw".

**Which issue(s) this PR fixes**:

**Special notes for your reviewer**:
This patch is not changing any code logic but it involves generated "code".

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