Improve javadoc to be compatible with more strict JDK8 javadoc standard

Build breaks with JDK8 because it fails in case of warnings when creating javadocs. Because with JDK8 javadoc is more strict and requires valid HTML.

See:
http://docs.oracle.com/javase/8/docs/technotes/guides/javadoc/whatsnew-8.html

One option would be to simply disable the warnings during the build an other option is to improve the docs.