document properties from getter @type

According to https://github.com/jsdoc3/jsdoc/issues/973, the right way to document getter/setter pairs is to document one of the two with a `@type`

It would be great if webcomponents.org would show types based on that comment.

Please see https://www.webcomponents.org/element/@power-elements/lazy-image/1.2.1/elements/lazy-image for an example of an element which documents getters that way. Property names and descriptions are shown, but types are shown as `?`