[SPARK-28464][DOC][SS] Document Kafka source minPartitions option
[SPARK-28464][DOC][SS] Document Kafka source minPartitions option

Adding doc for the kafka source minPartitions option to "Structured Streaming + Kafka Integration Guide"

The text is based on the content in  https://docs.databricks.com/spark/latest/structured-streaming/kafka.html#configuration

Closes #25219 from arunpandianp/SPARK-28464.

Authored-by: Arun Pandian <442b05a2ad153135def917e6a842adb2cfecb82d@groupon.com>
Signed-off-by: Dongjoon Hyun <9b02cb7e37158797a8e22c559f7c5f5ecd28ea57@apple.com>

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