Documentation updates

Summary:
Tackling a handful of docs issues:

- #12780
- #11227
- #2819
Closes https://github.com/facebook/react-native/pull/12867

Differential Revision: D4691083

Pulled By: hramos

fbshipit-source-id: 9115315f2d6132e975901c9a0b784e9d41eeb49e

docs/NativeModulesAndroid.md

@@ -283,60 +283,6 @@ measureLayout();
 
 Native modules should not have any assumptions about what thread they are being called on, as the current assignment is subject to change in the future. If a blocking call is required, the heavy work should be dispatched to an internally managed worker thread, and any callbacks distributed from there.
 
-### Sending Events to JavaScript
-
-Native modules can signal events to JavaScript without being invoked directly. The easiest way to do this is to use the `RCTDeviceEventEmitter` which can be obtained from the `ReactContext` as in the code snippet below.
-
-```java
-...
-private void sendEvent(ReactContext reactContext,
-                       String eventName,
-                       @Nullable WritableMap params) {
-  reactContext
-      .getJSModule(DeviceEventManagerModule.RCTDeviceEventEmitter.class)
-      .emit(eventName, params);
-}
-...
-WritableMap params = Arguments.createMap();
-...
-sendEvent(reactContext, "keyboardWillShow", params);
-```
-
-JavaScript modules can then register to receive events by `addListenerOn` using the `Subscribable` mixin
-
-```js
-import { DeviceEventEmitter } from 'react-native';
-...
-
-var ScrollResponderMixin = {
-  mixins: [Subscribable.Mixin],
-
-
-  componentWillMount: function() {
-    ...
-    this.addListenerOn(DeviceEventEmitter,
-                       'keyboardWillShow',
-                       this.scrollResponderKeyboardWillShow);
-    ...
-  },
-  scrollResponderKeyboardWillShow:function(e: Event) {
-    this.keyboardWillOpenTo = e;
-    this.props.onKeyboardWillShow && this.props.onKeyboardWillShow(e);
-  },
-```
-
-You can also directly use the `DeviceEventEmitter` module to listen for events.
-
-```js
-...
-componentWillMount: function() {
-  DeviceEventEmitter.addListener('keyboardWillShow', function(e: Event) {
-    // handle event.
-  });
-}
-...
-```
-
 ### Getting activity result from `startActivityForResult`
 
 You'll need to listen to `onActivityResult` if you want to get results from an activity you started with `startActivityForResult`. To do this, you must extend `BaseActivityEventListener` or implement `ActivityEventListener`. The former is preferred as it is more resilient to API changes. Then, you need to register the listener in the module's constructor,