docs: fix/add documentation for css.partial.createCSS on non-shadowed elements

[eljefe223]

🙋 Documentation Request

The documentation seems to be lacking on css.partial. Especially when importing and applying a css.partial to a non-shadowed element.

💁 Possible Solution

The solution is:

const myPartial = css.partial`color: red`;

.my-class {
    myPartial.createCSS(() => {});
}