Fill in RxSwift-specific information on ReactiveX doc pages

[DavidMGross]

The documentation got off to a good start with implementation-specific information for Java, Groovy, and JavaScript, but the equivalent RxSwift sections are mostly empty.

Pages that are in particular need of help here are:

The various operator pages linked to from http://reactivex.io/documentation/operators.html
http://reactivex.io/documentation/subject.html
http://reactivex.io/documentation/scheduler.html

[DavidMGross]

ping @carlosypunto