Docs: document data-turbo-boost-src usage

For example, inside a (nested) eager/lazy loaded `turbo-frame`, often you want to re-render that frame, not the entire page:

```html
<turbo-frame src="https://foo">
  <turbo-frame data-turbo-boost-src="https://foo" data-turbo-command="...">...</turbo-frame>
</turbo-frame>
```