[MINOR][DOC] Update the condition description of serialized shuffle

## What changes were proposed in this pull request?
`1. The shuffle dependency specifies no aggregation or output ordering.`
If the shuffle dependency specifies aggregation, but it only aggregates at the reduce-side, serialized shuffle can still be used.
`3. The shuffle produces fewer than 16777216 output partitions.`
If the number of output partitions is 16777216 , we can use serialized shuffle.

We can see this mothod: `canUseSerializedShuffle`
## How was this patch tested?
N/A

Closes #23228 from 10110346/SerializedShuffle_doc.

Authored-by: liuxian <liu.xian3@zte.com.cn>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleManager.scala

@@ -33,10 +33,10 @@ import org.apache.spark.shuffle._
  * Sort-based shuffle has two different write paths for producing its map output files:
  *
  *  - Serialized sorting: used when all three of the following conditions hold:
- *    1. The shuffle dependency specifies no aggregation or output ordering.
+ *    1. The shuffle dependency specifies no map-side combine.
  *    2. The shuffle serializer supports relocation of serialized values (this is currently
  *       supported by KryoSerializer and Spark SQL's custom serializers).
- *    3. The shuffle produces fewer than 16777216 output partitions.
+ *    3. The shuffle produces fewer than or equal to 16777216 output partitions.
  *  - Deserialized sorting: used to handle all other cases.
  *
  * -----------------------