chore: enhance API reference for SDK clients

[Priyanshu-u07]

Description

This PR improves the API reference documentation for Kubeflow SDK clients by
expanding docstrings with clearer descriptions, parameters, return types,
error details, and practical usage examples.

These documentation-only changes improve developer experience without
modifying any runtime behavior or public APIs.

Fixes #260

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[github-actions]

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