Test and document Image resizeMode=center on iOS

Summary:
`<Image resizeMode="center">` already works on iOS (implemented in #8792), but is neither tested nor documented the way the other `resizeMode` values are.

This PR primarily enables the relevant RNTester case on iOS, and secondarily copies over the doc comment from `Image.android.js` to `Image.ios.js`. A PR to `react-native-website` will follow shortly and it is there I will try and revise the wording a bit.

Updated RNTester screenshot (iOS):

<img src=https://user-images.githubusercontent.com/2246565/35470720-44b38282-0357-11e8-941c-1b3c5a1b2f3b.png width=300>

react-native-website PR coming soon.

[IOS] [MINOR] [Image] - Include resizeMode=center in RNTester
Closes https://github.com/facebook/react-native/pull/17759

Differential Revision: D6829051

Pulled By: hramos

fbshipit-source-id: c6e0000a75765e8bf3a1d0306aaafad002b14a58

Libraries/Image/Image.ios.js

@@ -241,6 +241,10 @@ const Image = createReactClass({
      *
      * - `repeat`: Repeat the image to cover the frame of the view. The
      * image will keep it's size and aspect ratio. (iOS only)
+     *
+     * - 'center': Scale the image down so that it is completely visible,
+     * if bigger than the area of the view.
+     * The image will not be scaled up.
      */
     resizeMode: PropTypes.oneOf([
       'cover',