[FLINK-1112] Add KeySelector group sorting on KeySelector grouping

The JavaDoc documentation needs some work, IMO.
KeySelectors are not restricted to Pojos and Tuples. They can extract a key from any data type.
I would phrase it as: "Sorts elements within a group on a key extracted by the specified KeySelector in the specified order."

The `@param order` refers to a field which has not been mentioned before. Should be the key extracted by the KeySelector.