Improve injectRoute docs (#3721)

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

src/content/docs/en/reference/integrations-reference.mdx

@@ -196,12 +196,24 @@ A callback function to inject routes into an Astro project. Injected routes can
 - `pattern` - where the route should be output in the browser, for example `/foo/bar`. A `pattern` can use Astro's filepath syntax for denoting dynamic routes, for example `/foo/[bar]` or `/foo/[...bar]`. Note that a file extension is **not** needed in the `pattern`.
 - `entryPoint` - a bare module specifier pointing towards the `.astro` page or `.js`/`.ts` route handler that handles the route denoted in the `pattern`.
 
-Example usage:
+##### Example usage
+
+```js
+injectRoute({
+  // Use Astro’s pattern syntax for dynamic routes.
+  pattern: '/subfolder/[dynamic]',
+  // Use relative path syntax for a local route.
+  entryPoint: './src/dynamic-page.astro'
+});
+```
+
+For an integration designed to be installed in other projects, use its package name to refer to the route entrypoint.
+The following example shows a package published to npm as `@fancy/dashboard` injecting a dashboard route:
 
 ```js
 injectRoute({
-  pattern: '/foo/[dynamic]',
-  entryPoint: 'foo/dynamic-page.astro'
+  pattern: '/fancy-dashboard',
+  entryPoint: '@fancy/dashboard/dashboard.astro'
 });
 ```