Update rss.mdx (#7878)

Co-authored-by: Luiz Ferraz <luiz@lferraz.com>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/guides/rss.mdx

@@ -214,6 +214,30 @@ export function GET(context) {
 }
 ```
 
+## Removing trailing slashes
+
+Astro's RSS feed produces links with a trailing slash by default, no matter what value you have configured for `trailingSlash`. This means that your RSS links may not match your post URLs exactly.
+
+If you have set `trailingSlash: "never"` on your `astro.config.mjs`, set `trailingSlash: false` in the `rss()` helper so that your feed matches your project configuration.
+
+```ts title="src/pages/rss.xml.js" ins={9}
+import rss from '@astrojs/rss';
+
+export function GET(context) {
+  const posts = Object.values(postImportResult);
+  return rss({
+    title: 'Buzz’s Blog',
+    description: 'A humble Astronaut’s guide to the stars',
+    site: context.site,
+    trailingSlash: false,
+    items: posts.map((post) => ({
+      link: post.url,
+      ...post.frontmatter,
+    })),
+  });
+}
+```
+
 ## Adding a stylesheet
 
 Style your RSS feed for a more pleasant user experience when viewing the file in your browser.