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Improve deleteOptions doc

**What this PR does / why we need it**:
This PR improves the doc of the 'PropagationPolicy' property of the DeleteOptions struct.
The said field is a string. It is very important a field for people who invoke a delete API.
For example, direct invocation of the deployment-delete API using the python client will have the replicaset and pods left behind. However, this field was not well documented. When people wanted to try different options, they have to dig into k8s source code to find out the acceptable values.

**Which issue this PR fixes**: fixes #

**Special notes for your reviewer**:

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