[Markdown] Format our returns as heading to match the params.

**If you're reporting a bug, please include _input code_, _output documentation_,
a description of what you expected to happen, and what happened instead.**

When the output is markdown, don't you think we should also format our returns as heading to match the params?
https://github.com/documentationjs/documentation/blob/master/src/output/markdown_ast.js#L181
https://github.com/documentationjs/documentation/blob/master/src/output/markdown_ast.js#L111

* What version of documentation.js are you using?: 14.0.2
* How are you running documentation.js (on the CLI, Node.js API, Grunt, other?):
Node.js API