docs: Modify legacy flag description (withastro#4187)

Co-authored-by: Sarah Rainsberger <[email protected]>

packages/astro/src/@types/astro.ts

@@ -724,6 +724,17 @@ export interface AstroUserConfig {
  * @description
  * Enable Astro's pre-v1.0 support for components and JSX expressions in `.md` Markdown files.
  * In Astro `1.0.0-rc`, this original behavior was removed as the default, in favor of our new [MDX integration](/en/guides/integrations-guide/mdx/).
+ * 
+ * To enable this behavior, set `legacy.astroFlavoredMarkdown` to `true` in your [`astro.config.mjs` configuration file](/en/guides/configuring-astro/#the-astro-config-file).
+ * 
+ * ```js
+ * {
+ * legacy: {
+ * // Example: Add support for legacy Markdown features
+ * astroFlavoredMarkdown: true,
+ * },
+ * }
+ * ```
  */
  astroFlavoredMarkdown?: boolean;
  };