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 closing `</p>` is not necessary for valid html and current  [javadoc documentation](http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html#format)  advertises the use of `<p>` without closing the paragraph explicitly with `</p>`:

> If you have more than one paragraph in the doc comment, separate the paragraphs with a <p> paragraph tag, as shown.

Please let me know if I still should adjust it.