update the note for importing a single file

Author: qiancai

tidb-cloud/import-csv-files-serverless.md

@@ -115,11 +115,11 @@ To import the CSV files to TiDB Cloud Serverless, take the following steps:
 
 5. In the **Destination Mapping** section, specify how source files are mapped to target tables.
 
-    The **Use [TiDB Dumpling file naming conventions](/tidb-cloud/naming-conventions-for-data-import.md) for automatic mapping** option is selected by default.
+    When you import multiple files, the **Use [TiDB Dumpling file naming conventions](/tidb-cloud/naming-conventions-for-data-import.md) for automatic mapping** option is selected by default.
 
     > **Note:**
     >
-    > When importing a single file, you can only use the default mapping settings, and the **Source** field cannot be modified.
+    > When you import a single file, the **Use [TiDB Dumpling file naming conventions](/tidb-cloud/naming-conventions-for-data-import.md) for automatic mapping** option is not displayed, and TiDB Cloud automatically populates the **Source** field with the file name. In this case, you only need to select the target database and table for data import.
 
     - To let TiDB Cloud automatically map all source files that follow the [TiDB Dumpling file naming conventions](/tidb-cloud/naming-conventions-for-data-import.md) to their corresponding tables, keep this option selected and select **CSV** as the data format.