Docs: Improve BackHandler documentation (addEventListener and removeEventListener)

Summary:
This commit adds documentation to two methods of BackHandler API, addEventListener and removeEventListener. Despite being a simple change, it helps to keep the documentation consistent.

I have tested the `./website` locally, the changes look similar to some other components such as `NetInfo`.

[DOCS][ENHANCEMENT][BackHandler] - Improve BackHandler documentation (addEventListener and removeEventListener)
Closes https://github.com/facebook/react-native/pull/16618

Differential Revision: D6260935

Pulled By: hramos

fbshipit-source-id: ab04a9fca89ddaa1925844b5754caf1c355a9329

Libraries/Utilities/BackHandler.android.js

@@ -74,6 +74,12 @@ var BackHandler = {
     DeviceEventManager.invokeDefaultBackPressHandler();
   },
 
+  /**
+   * Adds an event handler. Supported events:
+   *
+   * - `hardwareBackPress`: Fires when the Android hardware back button is pressed or when the
+   * tvOS menu button is pressed.
+   */
   addEventListener: function (
     eventName: BackPressEventName,
     handler: Function
@@ -84,6 +90,9 @@ var BackHandler = {
     };
   },
 
+  /**
+   * Removes the event handler.
+   */
   removeEventListener: function(
     eventName: BackPressEventName,
     handler: Function