Update CHANGES.md

Fix typos in Backpressure.md (ReactiveX#6927)

Fix lower case generic type declaration in reactive pull backpressure's code sample
Fix typo in the Further reading section

Release 2.2.18

More missing version tags

3.x: Add concatEagerDelayError across (ReactiveX#6899)

* 3.x: Add concatEagerDelayError across

* Add more since 3.0.0 tags

3.x: Simplify @return sentences in Flowable JavaDocs (ReactiveX#6847)

* 3.x: Simplify @return sentences in Flowable

* Clear Completable

* Clear Single

* Clean Maybe

* Clean Observable

Release 2.2.17

3.x: [Java 8] Add flattenStreamAsX to Maybe/Single (ReactiveX#6805)

* 3.x: [Java 8] Add flattenStreamAsX to Maybe/Single

* Add RS TCK tests for flattenStreamAsFlowable

Release 2.2.16

3.x: Change Flowable.groupBy to signal MBE instead of possibly hanging (

ReactiveX#6740)

* 3.x: Change Flowable.groupBy to signal MBE instead of possibly hanging

* Update groupBy javadocs

* Update backpressure annotation to ERROR, add some see links

* Correct javadocs typos

* Improve coverage of changes