docs: Add trailingComma ToString option

Author: eemeli

docs/03_options.md

@@ -166,4 +166,5 @@ Used by: `stringify()` and `doc.toString()`
 | nullStr                        | `string`                                                                                  | `'null'`  | String representation for `null` values.                                                                                                                                                                                      |
 | simpleKeys                     | `boolean`                                                                                 | `false`   | Require keys to be scalars and always use implicit rather than explicit notation.                                                                                                                                             |
 | singleQuote                    | `boolean ⎮ null`                                                                          | `null`    | Use 'single quote' rather than "double quote" where applicable. Set to `false` to disable single quotes completely.                                                                                                           |
+| trailingComma                  | `boolean`                                                                                 | `false`   | Add a trailing comma after the last entry in a flow collection that's split across multiple lines.                                                                                                                            |
 | trueStr                        | `string`                                                                                  | `'true'`  | String representation for `true` values.                                                                                                                                                                                      |