[FLINK-514]  Fixed javadoc problems in various problems

This pull requests fixes all javadoc warnings as reported by the maven build process except for the warnings from the flink-streaming-core module. flink-streaming-core references interfaces in the javadoc which do not exist anymore and it is not obvious to me how to change the javadoc.

The code changes in the pull request only affect javadoc. No actual program code has been touched.