+doc #15239: Stream cookbook
fixed merge conflicts, fixed reducebykey
=str #16549 doc: Integration with Reactive Streams
Merge pull request #16498 from drewhk/wip-15239-stream-cookbook-drewhk

Stream cookbook
+doc: Document graph cycles
Merge pull request #16579 from akka/wip-16549-doc-rs-integration-patriknw

=str #16549 doc: Integration with Reactive Streams
Merge pull request #16572 from drewhk/wip-stream-doc-graph-cycles-drewhk

+doc: Document graph cycles
+str #16464 generalised zipWith for up to 22 params
- removing deprecarted marker from japi, people will have to use it
  currently