SI-6580 Scaladoc: Should not close void elements

Because it will generate a useless element like "</img>".

To made matters worse, Scaladoc used to generate the element with
attributes (like </img src="...">). That's why we had SI-6580.
Merge pull request #2278 from kzys/jira-6580

SI-6580 Scaladoc: Should not close void elements