markdown writer should never wrap headings, but wraps them if they are too long

<h1><span><a href="https://qt.io/foo/bar/baz/lets/make/this/url/really/long/so/it/exceeds/eighty/columns">The Qt Web Site</a></span></h1>

->

# 
[The Qt Web Site](https://qt.io/foo/bar/baz/lets/make/this/url/really/long/so/it/exceeds/eighty/columns)

or

<H1>The Qt resource system is a platform-independent mechanism for shipping resource files in an application</H1>

->

# The Qt resource system is a platform-independent mechanism for shipping
resource files in an application