[MINOR][SQL][DOCS] Improve spark.sql.files.minPartitionNum's doc

### What changes were proposed in this pull request?

This PR aims to improve `spark.sql.files.minPartitionNum`'s doc.

### Why are the changes needed?

Improve description

### Does this PR introduce _any_ user-facing change?

Yes, better config description.

### How was this patch tested?

GA

Closes #40820 from Yikf/config-minPartitionNum-doc.

Authored-by: Yikf <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: yikf

docs/sql-performance-tuning.md

@@ -90,7 +90,7 @@ that these options will be deprecated in future release as more optimizations ar
     <td>Default Parallelism</td>
     <td>
       The suggested (not guaranteed) minimum number of split file partitions. If not set, the default
-      value is `spark.default.parallelism`. This configuration is effective only when using file-based
+      value is `spark.sql.leafNodeDefaultParallelism`. This configuration is effective only when using file-based
       sources such as Parquet, JSON and ORC.
     </td>
     <td>3.1.0</td>

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -1740,8 +1740,9 @@ object SQLConf {
 
   val FILES_MIN_PARTITION_NUM = buildConf("spark.sql.files.minPartitionNum")
     .doc("The suggested (not guaranteed) minimum number of split file partitions. " +
-      "If not set, the default value is `spark.default.parallelism`. This configuration is " +
-      "effective only when using file-based sources such as Parquet, JSON and ORC.")
+      s"If not set, the default value is `${LEAF_NODE_DEFAULT_PARALLELISM.key}`. " +
+      "This configuration is effective only when using file-based sources " +
+      "such as Parquet, JSON and ORC.")
     .version("3.1.0")
     .intConf
     .checkValue(v => v > 0, "The min partition number must be a positive integer.")