[SPARK-33840][DOCS] Add spark.sql.files.minPartitionNum to performence tuning doc

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

Add `spark.sql.files.minPartitionNum` and it's description to sql-performence-tuning.md.

### Why are the changes needed?

Help user to find it.

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

Yes, it's the doc.

### How was this patch tested?

Pass CI.

Closes #30838 from ulysses-you/SPARK-33840.

Authored-by: ulysses-you <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: ulysses-you

docs/sql-performance-tuning.md

@@ -85,6 +85,16 @@ that these options will be deprecated in future release as more optimizations ar
     </td>
     <td>2.0.0</td>
   </tr>
+  <tr>
+    <td><code>spark.sql.files.minPartitionNum</code></td>
+    <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
+      sources such as Parquet, JSON and ORC.
+    </td>
+    <td>3.1.0</td>
+  </tr>
   <tr>
     <td><code>spark.sql.broadcastTimeout</code></td>
     <td>300</td>