Update docs about supported native animation props

Summary:
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- [x] Explain the **motivation** for making this change.
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The docs had `backgroundColor` listed as a supported style prop for
native animation, but only `opacity` and `transform` are supported
at this time. See https://github.com/facebook/react-native/blob/master/Libraries/Animated/src/NativeAnimatedHelper.js#L108
My motivation was that I tried to use it and it didn't work. 😄

Github appears to render the change correctly.
Closes https://github.com/facebook/react-native/pull/13933

Differential Revision: D5071869

Pulled By: hramos

fbshipit-source-id: 947c8fa56b99d99c44603d8ef81664455234ed84

docs/Animations.md

@@ -356,7 +356,7 @@ You can also take a look at the [source code](https://github.com/facebook/react-
 
 Not everything you can do with `Animated` is currently supported by the native driver.
 The main limitation is that you can only animate non-layout properties:
-things like `transform`, `opacity` and `backgroundColor` will work, but flexbox and position properties will not.
+things like `transform` and `opacity` will work, but flexbox and position properties will not.
 When using `Animated.event`, it will only work with direct events and not bubbling events.
 This means it does not work with `PanResponder` but does work with things like `ScrollView#onScroll`.
 
@@ -490,4 +490,4 @@ option](http://facebook.github.io/react-native/blog/2017/02/14/using-native-driv
 You may also want to defer any computationally intensive work until after
 animations are complete, using the
 [InteractionManager](docs/interactionmanager.html). You can monitor the
-frame rate by using the In-App Developer Menu "FPS Monitor" tool.
+frame rate by using the In-App Developer Menu "FPS Monitor" tool.