Adds documentation for `Reactive` struct.
Adds documentation for `Reactive` struct.

shiratsu/RxSwift,ReactiveX/RxSwift,KyoheiG3/RxSwift,tottakai/RxSwift,kzaher/RxSwift,shiratsu/RxSwift,cfraz89/RxSwift,cfraz89/RxSwift,ReactiveX/RxSwift,kzaher/RxSwift,KyoheiG3/RxSwift,shiratsu/RxSwift,cfraz89/RxSwift,KyoheiG3/RxSwift,ReactiveX/RxSwift,cfraz89/RxSwift,kzaher/RxSwift,tottakai/RxSwift,tottakai/RxSwift,kzaher/RxSwift