1.x: add missing backpressure descriptions and update old ones (#4172)
1.x: add missing backpressure descriptions and update old ones (#4172)

* 1.x: add missing backpressure descriptions and update old ones

* Update based on suggestions

hzysoft/RxJava,weikipeng/RxJava,tilal6991/RxJava,nurkiewicz/RxJava,hzysoft/RxJava,tilal6991/RxJava,weikipeng/RxJava,tilal6991/RxJava,hzysoft/RxJava,davidmoten/RxJava,davidmoten/RxJava,weikipeng/RxJava,nurkiewicz/RxJava,davidmoten/RxJava,nurkiewicz/RxJava