add jsdoc for new option

src/index.ts

@@ -373,6 +373,16 @@ export interface CreateOptions {
    * For details, see https://nodejs.org/dist/latest-v18.x/docs/api/esm.html#customizing-esm-specifier-resolution-algorithm
    */
   experimentalSpecifierResolution?: 'node' | 'explicit';
+  /**
+   * Allow using voluntary `.ts` file extension in import specifiers.
+   *
+   * Typically, in ESM projects, import specifiers must hanve an emit extension, `.js`, `.cjs`, or `.mjs`,
+   * and we automatically map to the corresponding `.ts`, `.cts`, or `.mts` source file.  This is the
+   * recommended approach.
+   *
+   * However, if you really want to use `.ts` in import specifiers, and are aware that this may
+   * break tooling, you can enable this flag.
+   */
   experimentalTsImportSpecifiers?: boolean;
 }