Clarify the use case of Polaris Spark client

### Is your feature request related to a problem? Please describe.

We may clarify in the documentation that:
1. Do you need a Polaris Spark client to connect to Polaris?
No. For Iceberg tables, the standard Iceberg Spark client is sufficient.
Only Generic tables require the Polaris Spark client.
2. Classpath Isolation
Do not mix the Polaris Spark client and the Iceberg Spark client in the same classpath.
Each should run in its own isolated environment to avoid dependency conflicts.
3. Limitations of Polaris Spark Client

Related thread: https://apache-polaris.slack.com/archives/C084QSKD6S2/p1760817185391409

### Describe the solution you'd like

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### Describe alternatives you've considered

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### Additional context

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