[ReactNative] Small docs cleanup in ActivityIndicatorIOS and DatePickerIOS

Summary:
Summary:
Makes sure that it looks good in the website

Closes facebook#160
Github Author: Christopher Chedeau <vjeux@fb.com>

Test Plan:
Run the website

CC:

Task ID: #

Author: vjeux

Libraries/Components/ActivityIndicatorIOS/ActivityIndicatorIOS.ios.js

@@ -37,8 +37,11 @@ var ActivityIndicatorIOS = React.createClass({
      */
     color: PropTypes.string,
 
+    /**
+     * Size of the indicator. Small has a height of 20, large has a height of 36.
+     */
     size: PropTypes.oneOf([
-      'small', // default
+      'small',
       'large',
     ]),
   },

Libraries/Components/DatePicker/DatePickerIOS.ios.js

@@ -62,18 +62,16 @@ var DatePickerIOS = React.createClass({
 
     /**
      * The date picker mode.
-     *
-     * Valid modes on iOS are: 'date', 'time', 'datetime'.
      */
-    mode: PropTypes.oneOf(Object.keys(RCTDatePickerIOSConsts.DatePickerModes)),
+    mode: PropTypes.oneOf(['date', 'time', 'datetime']),
 
     /**
      * The interval at which minutes can be selected.
      */
     minuteInterval: PropTypes.oneOf([1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30]),
 
     /**
-     * Timezone offset in seconds.
+     * Timezone offset in minutes.
      *
      * By default, the date picker will use the device's timezone. With this
      * parameter, it is possible to force a certain timezone offset. For