message: docs(elements): add notice about choosing a tag name in Elements guide
  This commit adds a warning in the Elements guide about using
  `@Component.selector` as the tag name for the registered custom element.
  See also for context.
  PR Close
diff: @@ -71,6 +71,15 @@ When the browser encounters the tag for the registered element, it uses the cons
  <img src="generated/images/guide/elements/createElement.png" alt="Transform a component to a custom element" class="left">
  </div>
  +<div class="alert is-important">
  +
  + Avoid using the [`@Component`](api/core/Component) [selector](api/core/Directive#selector) as the custom-element tag name.
  + This can lead to unexpected behavior, due to Angular creating two component instances for a single DOM element:
  + One regular Angular component and a second one using the custom element.
  +
  +</div>
  +
  +
  ### Mapping
  A custom element _hosts_ an Angular component, providing a bridge between the data and logic defined in the component and standard DOM APIs. Component properties and logic maps directly into HTML attributes and the browser's event system.
  