docs: document showSummary option in article shortcode

exampleSite/content/docs/shortcodes/index.md

@@ -72,6 +72,8 @@ This is an error!
 | Parameter | Description                                              |
 | --------- | -------------------------------------------------------- |
 | `link`    | **Required.** the `.RelPermalink` to the target article. |
+| `showSummary` | **Optional.** A boolean value indicating whether to show the article summary. If not set, the site's default configuration will be used. |
+
 <!-- prettier-ignore-end -->
 
 **Example:**