Parse parameter and changelog entry descriptions with markdown

As described in the [WP Inline Docs standards](https://make.wordpress.org/core/handbook/inline-documentation-standards/php-documentation-standards/#other-descriptions), we're allowing some limited markdown in inline descriptions for various tags. Currently, that text is being passed through the runner absent markdown formatting.

This introduces a new `format_description()` function and relies on a new `line()` method introduced in Parsedown v1.0.0. Bumps Parsedown dependency to v1.1.4