Add note about astroHTML in .ts files

src/pages/en/guides/typescript.md

@@ -110,6 +110,16 @@ declare namespace astroHTML.JSX {
 }
 ```
 
+:::note
+`astroHTML` is injected globally inside `.astro` components. To use it in TypeScript files, use a [triple-slash directive](https://www.typescriptlang.org/docs/handbook/triple-slash-directives.html):
+
+```ts
+/// <reference types="astro/astro-jsx" />
+
+type MyAttributes = astroHTML.JSX.ImgHTMLAttributes;
+```
+:::
+
 ## Type checking
 
 To see type errors in your editor, please make sure that you have the [Astro VS Code extension](/en/editor-setup/) installed. Please note that the `astro start` and `astro build` commands will transpile the code with esbuild, but will not run any type checking. To prevent your code from building if it contains TypeScript errors, change your "build" script in `package.json` to the following: