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Update comments in pkg/kubeapiserver/authenticator/config.go

**What this PR does / why we need it**:
Make the comments consistent with the function signatures

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```
NONE
```