annotate hide-ui example (#2420)

Annotates the hide-ui example

### Change Type

- [ ] `patch` — Bug fix
- [ ] `minor` — New feature
- [ ] `major` — Breaking change
- [ ] `dependencies` — Changes to package dependencies[^1]
- [x] `documentation` — Changes to the documentation only[^2]
- [ ] `tests` — Changes to any test code only[^2]
- [ ] `internal` — Any other changes that don't affect the published
package[^2]
- [ ] I don't know

[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version

### Test Plan

1. Add a step-by-step description of how to test your PR here.
2.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- Annotate hide ui example

apps/examples/src/examples/hide-ui/HideUiExample.tsx

@@ -8,3 +8,9 @@ export default function HideUiExample() {
 		</div>
 	)
 }
+
+/* 
+This example shows how to hide the UI of the editor. Simply pass the hideUi prop
+to the Tldraw component. This is useful if you want to build your own UI around
+the editor. See the custom ui example if you need help building your own UI.
+*/