HTML attribute completion description

[octref]

Although we have no short descriptions for HTML attributes, custom contributions of HTML attributes can have completions.

In the future when https://github.com/mdn/short-descriptions expands to HTML, we can also add descriptions for all standard HTML attributes, too.

So we should implement the logic for showing attribute description in completion.

[octref]

This is already shipped.

Verifier: Use https://github.com/Microsoft/vscode-html-languageservice/blob/master/docs/customData.md to contribute some custom data, including an attribute with description field.

Here's an example: https://github.com/octref/svg-data

You should see attribute description showing up. Our built-in dataset don't have descriptions for all tags / attributes yet so this won't show up for common tags / attributes.