Further improvements to mdl-layout-icon documentation

Per @SeanBannister's [comment](https://github.com/google/material-design-lite/commit/7fd33e7b8ff692626537c5863fe6c0372b6f31d6#commitcomment-12614670):

> :) Thanks, now I understand how it work.
>
> Just a few things (I was going to do a PR for these but some might require discussion):
> 1. There's a few more div's in that doc using mdl-layout-icon. Was it intentional to only change the one?
> 2. `` is a void element, so it doesn't require that closing ``
> 3. Now that I understand how it works I wonder if the example in the doc should be an icon like `whatshot`. I just gave this a try and realised MDL doesn't vertically center the icon.
> 4. If an`` is used it'd be great if it could include a `src=""` to ensure its valid HTML and it'd also be a good visual example.
> 5. The example mdl-layout-icon is used on wouldn't actually show the icon because it display a menu icon.