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

Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 7ffc00c)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: HeartSaVioR

docs/structured-streaming-kafka-integration.md

@@ -379,10 +379,12 @@ The following configurations are optional:
   <td>int</td>
   <td>none</td>
   <td>streaming and batch</td>
-  <td>Minimum number of partitions to read from Kafka.
+  <td>Desired minimum number of partitions to read from Kafka.
   By default, Spark has a 1-1 mapping of topicPartitions to Spark partitions consuming from Kafka.
   If you set this option to a value greater than your topicPartitions, Spark will divvy up large
-  Kafka partitions to smaller pieces.</td>
+  Kafka partitions to smaller pieces. Please note that this configuration is like a `hint`: the
+  number of Spark tasks will be **approximately** `minPartitions`. It can be less or more depending on
+  rounding errors or Kafka partitions that didn't receive any new data.</td>
 </tr>
 </table>