clarify documentation on cache()
clarify documentation on cache()

joohnnie/RxScala,zhongdj/RxScala,zjrstar/RxScala,TracyLu/RxScala,ReactiveX/RxScala,samuelgruetter/RxScala,hermanbanken/RxScala,jbripley/RxScala,hermanbanken/RxScala,samuelgruetter/RxScala,jbripley/RxScala