Improve documentation for importing HTML or JavaScript

[OlliTietavainenVaadin]

In the docs "Importing JavaScript, JavaScript modules, and HTML" there should be a more complete example of what you can do with the HTML and the JavaScript in the following sample:

@Tag("div")
@JsModule("./src/my-module.js")
@JavaScript("/js/script.js")
@HtmlImport("/html/htmlimport.html")
static class CustomComponent extends Component
        implements HasText {
  // implementation omitted
}