Identify keyboardDismissMode platform-specific options

Summary:
Similar to `TextInput`'s `returnKeyType`, comments allow to see at a glance which options are cross-platform and which are not.

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Closes https://github.com/facebook/react-native/pull/14780

Differential Revision: D5480895

Pulled By: hramos

fbshipit-source-id: c38337def920678d29c8322e52b54f57e80cb95b

Libraries/Components/ScrollView/ScrollView.js

@@ -201,16 +201,21 @@ const ScrollView = createReactClass({
     /**
      * Determines whether the keyboard gets dismissed in response to a drag.
      *
+     * *Cross platform*
+     *
      *   - `'none'` (the default), drags do not dismiss the keyboard.
      *   - `'on-drag'`, the keyboard is dismissed when a drag begins.
+     *
+     * *iOS Only*
+     *
      *   - `'interactive'`, the keyboard is dismissed interactively with the drag and moves in
      *     synchrony with the touch; dragging upwards cancels the dismissal.
      *     On android this is not supported and it will have the same behavior as 'none'.
      */
     keyboardDismissMode: PropTypes.oneOf([
       'none', // default
-      'interactive',
-      'on-drag',
+      'on-drag', // Cross-platform
+      'interactive', // iOS-only
     ]),
     /**
      * Determines when the keyboard should stay visible after a tap.