[addon-docs] Newlines not rendered in markdown code blocks in prop comments

Describe the bug When adding code blocks to a docblock in markdown format, they appear in Storybook as a single line instead of properly formatted.

To Reproduce Add the following comment to a propType declaration:

/**
 * ```
 * classNames={{
 *   appear: 'my-appear',
 *   appearActive: 'my-active-appear',
 *   appearDone: 'my-done-appear',
 *   enter: 'my-enter',
 *   enterActive: 'my-active-enter',
 *   enterDone: 'my-done-enter',
 *   exit: 'my-exit',
 *   exitActive: 'my-active-exit',
 *   exitDone: 'my-done-exit',
 * }}
 * ```
 */

Expected behavior It should appear in Storybook as follows:

classNames={{
  appear: 'my-appear',
  appearActive: 'my-active-appear',
  appearDone: 'my-done-appear',
  enter: 'my-enter',
  enterActive: 'my-active-enter',
  enterDone: 'my-done-enter',
  exit: 'my-exit',
  exitActive: 'my-active-exit',
  exitDone: 'my-done-exit',
}}

Actual Behavior It appears as:

classNames={{ appear: 'my-appear', appearActive: 'my-active-appear', appearDone: 'my-done-appear', enter: 'my-enter', enterActive: 'my-active-enter', enterDone: 'my-done-enter', exit: 'my-exit', exitActive: 'my-active-exit', exitDone: 'my-done-exit', }}

System:

Environment Info:

  System:
    OS: macOS 10.15.5
    CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
  Binaries:
    Node: 12.16.1 - ~/.nvm/versions/node/v12.16.1/bin/node
    npm: 6.14.5 - ~/Desktop/react-skeleton-generator/waltz/node_modules/.bin/npm
  Browsers:
    Chrome: 83.0.4103.116
    Firefox: 77.0.1
    Safari: 13.1.1
  npmPackages:
    @storybook/addon-a11y: ^5.3.19 => 5.3.19
    @storybook/addon-actions: ^5.3.19 => 5.3.19
    @storybook/addon-docs: ^5.3.19 => 5.3.19
    @storybook/addon-knobs: ^5.3.19 => 5.3.19
    @storybook/addon-links: ^5.3.19 => 5.3.19
    @storybook/addon-viewport: ^5.3.19 => 5.3.19
    @storybook/addons: ^5.3.19 => 5.3.19
    @storybook/react: ^5.3.19 => 5.3.19

[niklasmh]

@shilman Superb! I am more than happy to do an attempt 😄

[shilman]

Jeepers creepers!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.4.0-alpha.31 containing PR #12882 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease