Docs: Update TouchableWithoutFeedback.js

Summary:
Add description for onPressIn and onPressOut.

Here is the snack to illustrate it.
https://snack.expo.io/Byed5cKBW

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Closes https://github.com/facebook/react-native/pull/15045

Differential Revision: D5434900

Pulled By: hramos

fbshipit-source-id: b235c3649e63b0bd149b0a65e439cd2433b01b8a

Libraries/Components/Touchable/TouchableWithoutFeedback.js

@@ -59,8 +59,15 @@ const TouchableWithoutFeedback = createReactClass({
      * that steals the responder lock).
      */
     onPress: PropTypes.func,
+    /**
+    * Called as soon as the touchable element is pressed and invoked even before onPress.
+    * This can be useful when making network requests.
+    */
     onPressIn: PropTypes.func,
-    onPressOut: PropTypes.func,
+    /**
+    * Called as soon as the touch is released even before onPress.
+    */
+     onPressOut: PropTypes.func,
     /**
      * Invoked on mount and layout changes with
      *