docs: mention special case for trailing slash (#74839)

### What

Clarify trailing slash special cases
[slack-ref](https://vercel.slack.com/archives/C04DUD7EB1B/p1718055917076779)

docs/01-app/04-api-reference/05-config/01-next-config-js/trailingSlash.mdx

@@ -17,6 +17,13 @@ module.exports = {
 
 With this option set, URLs like `/about` will redirect to `/about/`.
 
+When using `trailingSlash: true`, certain URLs are exceptions and will not have a trailing slash appended:
+
+- Static file URLs, such as files with extensions.
+- Any paths under `.well-known/`.
+
+For example, the following URLs will remain unchanged: `/file.txt`, `images/photos/picture.png`, and `.well-known/subfolder/config.json`.
+
 When used with [`output: "export"`](/docs/app/building-your-application/deploying/static-exports) configuration, the `/about` page will output `/about/index.html` (instead of the default `/about.html`).
 
 ## Version History