Fix onKeyPress documentation of TextInput

Summary:
Only tested on iOS.
Closes https://github.com/facebook/react-native/pull/11621

Differential Revision: D4374415

Pulled By: mkonicek

fbshipit-source-id: b686d0480b8db181dae46a94e9d1b88eab488074

Libraries/Components/TextInput/TextInput.js

@@ -376,7 +376,9 @@ const TextInput = React.createClass({
     onSubmitEditing: PropTypes.func,
     /**
      * Callback that is called when a key is pressed.
-     * Pressed key value is passed as an argument to the callback handler.
+     * This will be called with `{ nativeEvent: { key: keyValue } }`
+     * where `keyValue` is `'Enter'` or `'Backspace'` for respective keys and
+     * the typed-in character otherwise including `' '` for space.
      * Fires before `onChange` callbacks.
      * @platform ios
      */