Fix the Javadoc link in DataStream class

Fix the Javadoc link for org.apache.flink.api.common.functions.RichFunction and missing method reference in DataStream class.

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #154 from hsaputra/fix_comments_streaming and squashes the following commits:

127e47a [Henry Saputra] Fix the Javadoc link for org.apache.flink.api.common.functions.RichFunction and missing method reference.