Improve documentation of `ifEmpty(default:)` operator
Improve documentation of `ifEmpty(default:)` operator

ReactiveX/RxSwift,ReactiveX/RxSwift,cfraz89/RxSwift,kzaher/RxSwift,kzaher/RxSwift,cfraz89/RxSwift,kzaher/RxSwift,ReactiveX/RxSwift,cfraz89/RxSwift,cfraz89/RxSwift,kzaher/RxSwift