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

[HeartSaVioR]

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.

spark/external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaOffsetRangeCalculator.scala

Lines 32 to 46 in d67b98e

	/**
	* Calculate the offset ranges that we are going to process this batch. If `minPartitions`
	* is not set or is set less than or equal the number of `topicPartitions` that we're going to
	* consume, then we fall back to a 1-1 mapping of Spark tasks to Kafka partitions. If
	* `numPartitions` is set higher than the number of our `topicPartitions`, then we will split up
	* the read tasks of the skewed partitions to multiple Spark tasks.
	* The number of Spark tasks will be *approximately* `numPartitions`. It can be less or more
	* depending on rounding errors or Kafka partitions that didn't receive any new data.
	*
	* Empty ranges (`KafkaOffsetRange.size <= 0`) will be dropped.
	*/
	def getRanges(
	fromOffsets: PartitionOffsetMap,
	untilOffsets: PartitionOffsetMap,
	executorLocations: Seq[String] = Seq.empty): Seq[KafkaOffsetRange] = {

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.

[HeartSaVioR]

cc. @dongjoon-hyun @zsxwing

[SparkQA]

Test build #108535 has finished for PR 25332 at commit ffe033c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

+1, LGTM. Thank you for this follow-up, @HeartSaVioR !
Merged to master/branch-2.4.

[HeartSaVioR]

Thanks for the quick review and merge!