Summary:
Sending global events with `eventDispatcher` has been [deprecated](d9737571c4) since 0.28, with a recommendation to subclass `RCTEventEmitter` instead, but there were no docs for this.
This updates the iOS native module to use the recommended way, and also documents the use of `stopObserving` and `startObserving`.
Tested and working in a real app.
cc nicklockwood
Closes https://github.com/facebook/react-native/pull/11792
Differential Revision: D4394626
fbshipit-source-id: 68b26d35944a521bf683a50ec1ab21f03b5e99d8