This PR expands the `HtmlElementView` widget DartDocs, with the following sections:

* Usage: How to use the widget, two ways:
  * The `HtmlElementView.fromTagName` constructor
  * The `PlatformViewRegistry` way
* Lifecycle: There's an `onCreated` callback on the widget. When does it get called?
* HTML Lifecycle: How to listen to events coming from the DOM.
* Visibility: what is the `isVisible` property for?

Small additional tweaks here and there to mention common pitfalls of using HtmlElementView on the web, and mentions to workarounds, like `package:pointer_interceptor`.

## Issues

* Fixes: https://github.com/flutter/flutter/issues/143922
* Fixes: https://github.com/flutter/flutter/issues/49634
* Related: https://github.com/flutter/website/issues/5063