dumpling: clarify description about --csv-separator (#22098) (#22100)

Author: ti-chi-bot

dumpling-overview.md

@@ -406,7 +406,7 @@ SET GLOBAL tidb_gc_life_time = '10m';
 | `--cert`                     | The address of the client certificate file for TLS connection                                                                                                                                                                                                                                                                      |
 | `--key`                      | The address of the client private key file for TLS connection                                                                                                                                                                                                                                                                      |
 | `--csv-delimiter`            | Delimiter of character type variables in CSV files                                                                                                                                                                                                                                                                                 | '"'                                        |
-| `--csv-separator`            | Separator of each value in CSV files. It is not recommended to use the default ','. It is recommended to use '\|+\|' or other uncommon character combinations| ','                                                                                                                                                                                                                                                                                               | ','                                        |
+| `--csv-separator`            | Separator for each value in CSV files. If your data contains commas, it is recommended to use a combination of uncommon characters as the separator. Invisible characters are also supported, for example: `--csv-separator $'\001'`. | ','                                        |
 | `--csv-null-value`           | Representation of null values in CSV files                                                                                                                                                                                                                                                                                         | "\\N"                                      |
 | `--escape-backslash`         | Use backslash (`\`) to escape special characters in the export file                                                                                                                                                                                                                                                                | true                                       |
 | `--output-filename-template` | The filename templates represented in the format of [golang template](https://golang.org/pkg/text/template/#hdr-Arguments) <br/> Support the `{{.DB}}`, `{{.Table}}`, and `{{.Index}}` arguments <br/> The three arguments represent the database name, table name, and chunk ID of the data file                                  | '{{.DB}}.{{.Table}}.{{.Index}}'            |