Update documentation for Modal's prop animationType

Summary:
I was using Modal component and I didn't knew what was the default for animationType prop. After reading the code, I saw that it was `none`.

I did not tell that the "default" is `slide` if animated is set to `true`, because it is not a default but an implementation of `animated` prop effect and because of the deprecation of `animated`.

Thanks.
Closes https://github.com/facebook/react-native/pull/13196

Differential Revision: D4795765

Pulled By: hramos

fbshipit-source-id: 25d62ac7eeb20fc5557918c3d75709f44f5d4972

Libraries/Modal/Modal.js

@@ -87,6 +87,8 @@ class Modal extends React.Component {
      * - `slide` slides in from the bottom
      * - `fade` fades into view
      * - `none` appears without an animation
+     *
+     * Default is set to `none`.
      */
     animationType: PropTypes.oneOf(['none', 'slide', 'fade']),
     /**