Addon-docs: Add docs.description parameter

[shilman]

Issue: #11441

What I did

import { Button } from './Button';

export default {
  title: 'Button',
  parameters: {
    docs: { description: { component: 'some component **markdown**' } }
  }
}

export const Basic = () => <Button />
Basic.parameters = {
  docs: { description: { story: 'some story **markdown**' } }
}

How to test

See updated example in official-storybook

[tmeasday]

LGTM