[FLINK-2818] [runtime] Corrected javadocs of *ReduceDriver classes

[ggevay]

• "The iterator is handed to the reduce() method of the ReduceFunction."
  In the cases of ReduceDriver and AllReduceDriver, it is not the iterator which is handed to reduce(), but the elements pairwise
• "The iterator returns all records grouped by their key"
  I guess this is not the case with AllReduceDriver and AllGroupReduceDriver. But someone who knows the code please confirm this!
• AllGroupReduceDriver refers to "RichGroupFunction"
  There is no such thing. I guess this should be RichGroupReduceFunction.
• GroupCombineChainedDriver refers to "GroupCombineDriver"
  There is no such thing. I guess this should be GroupReduceCombineDriver? (please confirm)
• *Driver instead of *Task
• In the combineGroup methods in DataSet, SortedGrouping, and UnsortedGrouping, the text of the NullPointerException is wrong.
• The @return annotation of the first() method refers to "ReduceGroupOperator" instead of "GroupReduceOperator".
• The javadoc of combineGroup() refers to CombineFunction instead of GroupCombineFunction
• minor typos
• CombineFunction is not used anywhere (except in comments), but I didn't do anything about this, because I'm not sure whether it should be removed, or perhaps there are plans for using it in the future.

[StephanEwen]

Looks good, +1 to merge

BTW: I think its not necessary to {@link} the return types - they are linked anyways in the JavaDoc.

[mxm]

+1 to merge.

[chiwanpark]

I'll merge this with addressing @StephanEwen's comment.