SSR guide: add directive transforms

The Documentation for writing SSR Transforms for directives, beside a very basic noop example, is nonexistent. I've written an Image Lazy Loading Directive and want to provide an SSR Transform. adding simple properties like src or poster are working flawless. When it comes to adding a style attribute the lack of documentation makes it very hard find a solution. The transform isn't very complex, but i'm struggling to add style='background-image:url()". See the Transform code below

export const vueLazyLoaderSSRTransform = (dir, node, context) => {
    if (node.tag === 'img') {
        return {
            props: [createObjectProperty('src', dir.exp)]
        }
    }

    if (node.tag === 'video') {
        return {
            props: [createObjectProperty('poster', dir.exp)]
        }
    }

    return {
        props: [
            createObjectProperty('style',
                createObjectExpression([
                    createObjectProperty(
                        'background-image',
                        createSimpleExpression('url()', true)
                    )
                ])
            )
        ]
    }
}

It would be great to have a more in depth documentation on SSR Transforms

Start by reviewing the existing basic noop SSR transform example and the directive transform code included in the issue. Document how SSR directive transforms handle properties such as src, poster, and style, including the reported background-image case; done means a newcomer can follow the guide to implement a comparable transform.