Fix JDK8 javadoc issues

Fixed errors when running mvn site on JDK8.
These were caused by the backwards-compatibility breaking changes of javadoc in JDK8.
The HTML specification defines the manner in which a browser should infer the location of the closing tag, which works fine for Javadoc in isolation. However, some of the automated translation tools used for creating other ANTLR language targets look for closing tags (for one reason or another), so for this project we've made it a practice to explicitly close all tags.