Merge pull request #691 from Netflix/docs

javadoc improvements:
Kotlin M6.2
Merge pull request #693 from MarioAriasC/master

Kotlin M6.2
Change in wiki page from "Mathematical Operators" to "Mathematical and Aggregate Operators"
move toList, reduce, concat, toSortedList, toMap, and toMultiMap to that page
deprecate aggregate in favor of reduce
[should I move takeLast, last, and takeLastBuffer to the Aggregate operators page? they don't operate on the aggregate, exactly, but they do wait until the source completes before emitting anything]