Add comments for memory-related reduce operations.

Resolve #1000. 
I found this documentation useful myself. Since when I was choosing sorting approaches, I would like to know which uses a in-memory heap and which uses external sorting.  

aka 'secondary sorting' in Hadoop. Could say 'All sorting methods defined here trigger Hadoop secondary sort on value. i. e. won't materialize all values of each key in memory on the reducer'. 

I should add a description to recent 'qna' expanding on answer that Oscar gave about using sortWithTake().