CSF Stories with MDX Docs does not take decorators from csf stories

[aizerin]

Describe the bug
When defining decorators in CSF stories then they are not applied in MDX. Which is wrong according to

https://github.com/storybookjs/storybook/blob/next/addons/docs/docs/recipes.md#csf-stories-with-mdx-docs

To Reproduce
Steps to reproduce the behaviour:

1. Create story by recipe mentioned above and add decorator
2. tart storybook
3. See the docs tabs

Expected behaviour
Decorator will be applied when using story in MDX.

Screenshots

Code snippets

import React from "react";
import { Button } from "@storybook/react/demo";

export default {
  title: "a/mix",
  includeStories: []
};

export const basic = () => <Button>Hello Button</Button>;

basic.story = {
  decorators: [storyFn => <div>I'm decorated {storyFn()}</div>]
};

import { Meta, Story, Preview } from "@storybook/addon-docs/blocks";
import * as stories from "./mix.stories.js";

<Meta title="a/mix" />

# Button

<Preview>
  <Story name="basic">{stories.basic()}</Story>
</Preview>

System:

  System:
    OS: macOS 10.15.2
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  Binaries:
    Node: 12.13.1 - ~/.nvm/versions/node/v12.13.1/bin/node
    Yarn: 1.21.1 - /usr/local/bin/yarn
    npm: 6.12.1 - ~/.nvm/versions/node/v12.13.1/bin/npm
  Browsers:
    Chrome: 79.0.3945.130
    Firefox: 70.0.1
    Safari: 13.0.4
  npmPackages:
    @storybook/addon-actions: ^5.3.7 => 5.3.7
    @storybook/addon-docs: ^5.3.7 => 5.3.7
    @storybook/addon-links: ^5.3.7 => 5.3.7
    @storybook/addons: ^5.3.7 => 5.3.7
    @storybook/react: ^5.3.7 => 5.3.7

[shilman]

Yeah, you'll need to add those decorators yourself in MDX.

[aizerin]

Ok, so this sentence from docs is no longer valid? I can update it.

The MDX loader is using story metadata from CSF, such as name, decorators, parameters, but will give giving preference to anything defined in the MDX file.

[shilman]

Ouch. That section is wrong. 🙈 I'll update it now, can you review the PR for me?

[aizerin]

Sure