java-client: fix some javadoc comments

[rubenhoenle]

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg

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[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/17588981849/job/49970779927?pr=21932 not related to this change