[MINOR][DOC][SS] Correct description of minPartitions in Kafka option
[MINOR][DOC][SS] Correct description of minPartitions in Kafka option

## What changes were proposed in this pull request?

`minPartitions` has been used as a hint and relevant method (KafkaOffsetRangeCalculator.getRanges) doesn't guarantee the behavior that partitions will be equal or more than given value.

https://github.com/apache/spark/blob/d67b98ea016e9b714bef68feaac108edd08159c9/external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaOffsetRangeCalculator.scala#L32-L46

This patch makes clear the configuration is a hint, and actual partitions could be less or more.

## How was this patch tested?

Just a documentation change.

Closes #25332 from HeartSaVioR/MINOR-correct-kafka-structured-streaming-doc-minpartition.

Authored-by: Jungtaek Lim (HeartSaVioR) <71c146bf4d269d28958f88a73164915cec8cbc61@gmail.com>
Signed-off-by: Dongjoon Hyun <9b02cb7e37158797a8e22c559f7c5f5ecd28ea57@apple.com>

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